What TV series essentially became unwatchable to you after one decision by the writers?

Anything that's been "adapted" in the last decade or so: Witcher? Dog shit. Halo? Hell no. Wheel of Time? Fuck off. Amazon LoTR? Fucking spare me. Disney ANYTHING?!? I'd rather set myself on fire than validate the raping of all my childhood heroes.

Writers are an extinct species. All we have now are cultural vandals.

Most of human history has been this way. People telling and re-telling their own fanfiction or alternate versions of everything from Greek tragedies to the Bible to everything else.
There have been good adaptations. The Castlevania anime was great. The first one at least.
I think the bigger issue is the trend of putting a lot of money and effort into season 1 to make a good impression and try to get loyal fans, then to take all the money out for subsequent seasons and hope the fans stick around.
It's interesting that you mention the Witcher because I thought season 1 was way better than anything the games did. I could see how the structure in particular might not be for everyone. After season 1 it's obvious that everything went cheap- the lighting, the costumes, the makeup, the locations, the editing, the sound design, the CG effects, the writing. I normally don't even have an appreciation for costume design, but by season 3 it looked like they went to Spirit for Geralt's armor and Kohl's for Yennifer's dresses.
Arcane was another one. Season 1 was way better than anything League of Legends did during the few years I played. Season 2 was still... Okay, but the writing was terrible. The backgrounds went from detailed panoramas of piltover to just stylized colored fog. A lot of the characters seem to do things that go against their previously established motives or just do things that don't actually make sense but are required to move the plot along.
To bring it back to Castlevania- after the original series, the Nocturne series felt like a cheap knockoff. Which was a real shame because it was the same studio and I really enjoyed most of their other work.
There have been some good adaptations though, even ones that did their own thing - like Foundation.
Castlevania is fucking sick tho. Watchmen the series is arguably equivalent to the OG comic in terms of writing and artistic quality.
There are genuinely great adaptations out there.

Game of Thrones.

Which one of their many bad decisions?
- Anything in the last 2 seasons?

The Boys. Season 3. If the season finale had been episode 6 I would have returned, but the final 2 episodes went more Marvel than Marvel being so afraid to kill Homelander or Butcher. It made the plot armor way too had to ignore, and became obvious they were just going to drag things out for multiple seasons.

Finally something I'm qualified to answer!

Family Guy - While more "Flanderization" when all the characters became parodies of their previous selves, the real thing is when all of a sudden every joke needed to be "explained". i.e. someone trips while running and then one of the character says "AHHH HE TRIPPED AND HE FELL". Ruined every joke for me, was done with it after that. Kind of one decision, I'm sure someone said "people don't get the jokes" and ruined the show for me.

House - When they suddenly swapped the cast out. Some people really liked it, for me that was the end of the original premise of the show.

How I Met Your Mother - Lily and Marshall's baby. Having a baby in a show is always kind of the "Okay the romance story is over and the writers are out of ideas" but it was done so badly. Lily was always an extremely selfish character, and she was insufferable through everything, from making the literal choice to have a child based on some fated chance to shaming Marshall for having the gall to - go to work to support all of them while she did... nothing? Did she even have a job? Constant guilt trips and manipulation, she was the worst character but their relationship was just straight-up toxic when she got pregnant.

That 70's Show - Having Donna stay home from college - 4 years of characterization thrown out the window. She was always a strong independent woman and now she's just apparently throwing away her future? She attends remote classes for a while then it just disappears. Then suddenly Eric has this big revelation later that him marrying her might lead to an unsatisifed future? It was all just very clear that the writers didn't know how to handle her being gone - a very common coming of age issue

X-Files - The move to California. Enough said.

On a smaller and younger scale it was like this with the Super Friends (justice league).
Superman: "I'm going to punch this asteroid that was on a direct course to destroy metropolis which will break it in half, one half fills the gap in a bridge a train is about to cross and the other plugs a hole in a dam"
superman proceeds to do just that and it pissed very young me off.
- The Super Friends was a wild show. There's an episode where they fight the literal fucking Titanic and kill it by tricking it into running into another iceberg. I'm not joking or exaggerating.
- That sounds hilarious

Happy Days when they had The Fonz water ski jump over a shark.

Yep, I'm old, and yes that is where the term "Jump the shark" is from.

Except that isn’t true.
Yes the show peaked in the ratings the season Fonzie jumped the shark, but the show went on for another 8 years and the later seasons were when the show was better than ever.
Also the phrase comes from an interviewee on the Howard Stern show, talking about Happy Days, 20 years after the show ended.
- That is when the show became so dumb I did not watch anymore, and it is what the phrase is from, I didn't say who coined it or when.
  Also, Jon Hein coined the term in 1985, 8 years after the episode aired https://en.wikipedia.org/wiki/Jumping_the_shark
I remember seeing an interview with Tom Bosley, the actor who played Mr. Cunningham, where he points out that there were more episodes of Happy Days after jumping the shark than before, so it was hardly the death knell for the show.
- Simpsons also has more seasons after they jumped the shark than before.
- Yes, but that is the point it was done for me.

Not sure if it counts, but I loved the Lord of the Rings movies. Had high hopes for The Hobbit, but the way they portrayed Thorin completely broke it for me. Didn't bother watching the other two.

Cue Lindsay Ellis
You didnt miss much. They were very forgettable.

Controversial take but Star Trek Voyager.

When I was a kid I loved it, it was my favourite Star Trek. then they introduced Seven of Nine and suddenly it became a borgfest.

I liked it because it was all new alien races, stuff that's NEVER been in Star Trek before...and then they were like "no ones watching, lets bring in the Borg."

I think you mean Seven of Fine
As I recall, the first introduction of the Borg was in the Delta Quadrant (when Q tossed the Enterprise there as a lark). So not including them would seem like a huge oversight considering their dominance.
This was also after First Contact came out, I assume that had something to do with it.
There was also still a huge host of episodes that had nothing to do with borg, aside from the occasional "We need to improve this technology because plot reasons, let's throw some borg nanites in it or something". It wasn't wholly focused on the borg.
Disagree
I have the opposite opinion. Voyager was my first trek as a kid as well. Upon rewatching it, the show actually isn’t great until seven shows up. It kinda drags ass until season 4.
Interesting. DS9 would’ve been more to your liking then. The Borg were only in the first episode as one or two scenes to set up Sisko’s character arch. After that, it’s mostly new races, and more advanced development of existing boring races (i.e. Ferringhi, Bajorians).

Jessica Jones. Season one was fucking magnificent. Season 2+ is.... A second season of that show. My understanding is, that they changed the writing staff of the show.

The end of season one set up season two to be a hard boiled detective show. That would have been cool. Stand alone episodes with an overlying story arc

the 100 after that moment

Wth happened I stopped watching bc I got bored

Dexter. 1 ep after trinity, I stopped. Decided lithgow had a perfect ending.

It really was the peak of what i remember from the show. It does having an intriguing consequence that Dexter starts to be concerned his son will have the same reaction that Dexter did to witnessing his mother's death and starts monitoring him for signs. If I remember correctly, he's both concerned that his son will be cursed to be like him, but also excited by the idea of having a bond and mentorship with him through it. But even that gets a bit hamfisted as I recall and ultimately goes nowhere. The show really whimpered out in later seasons.

The 2nd episode of "Man in the High Castle" when you realize they just copied the setting and are going to ignore the key elements (and strengths) of the book.

I gave up on the adaption about 5 min into the second episode.

I enjoyed that show immensely, until the rushed last season. I didn’t read the book though.
- I actually liked the series better than the book. And I generally like PKD.

Mr. Robot. I loved the pilot episode so much, but by episode 3 or 4 it became glaringly obvious to me that the writers didn't understand computers.

That is a highly unusual comment regarding Mr. Robot

I started watching 'The 100.'

I gave up when the folks from orbit decided that they had to massacre the ground dwellers.

This was after a dozen of so other massacres.

I regret sticking with it until the end. Such a garbage ending.
- Thanks. It's nice to know that I didn't miss anything.
  If you haven't started it yet, look for "Pluribus." That's a good SF show

Eureka. The time travel, and that god fucking damn bunker

South Park lost their ways in season 27 and 28. It's hard to say since it's one of my favorite shows.

I haven't watched it, but I'd say losing your way in season 27 is pretty damn impressive. It's like someone living healthily to 110.
I felt the opposite. They lost me a few seasons ago, especially with the weird season/movie thing. But I really felt 27 was a return to form, especially Got a Nut’s classic A & B plot convergence and the overall return to kid antics as plot devices.

Not really decisions by the writers, but more losing the good cast members.

Misfits and Being Human UK

Bob's Burgers around season 5 or 6. The tone changed and it was less learning about some ratchet ass setting and meeting the people in it and more about sticking to the established normalized rules for the setting. Mort kinda vanished.

Doctor who. Every decision after Capaldi.

What do you mean? Capaldi is still the Doctor and no one will convince me otherwise.
- 😂
My problems with the show started earlier, still during Tennant's run. They kept writing scenes where his companion would gush about how great he is, and The Doctor would brag about himself in a way that didn't feel like a character flaw. I preferred the writing when he would make a good Dalek, when Rose had the realization she'd been kidnapped by a fae creature. Everything's gotten so chipper and quirky, and I don't feel like the writing is thought about very deeply.
The thing is, I love Jodie Whittaker and was so excited for her Doctor. But her companions were so bland. Ugh just super boring. And I couldn't keep up with the lore for some reason. It just didn't hold my interest and I was so sad about it. :(
That's when they brought on the showrunner for Torchwood. Was it any wonder it got less fanciful and more melodramatic?
I can pinpoint the moment, it was "solitract." Like, lonely intestines? I think I finished out that season, but haven't watched any Doctor Who since then. It was just so hand-wavey with a ridiculously dumb name that it demolished my ability to suspend disbelief.

I wouldn't say "unwatchable" but one thing I really didn't like about the King of the Hill revival was aging everyone up. Mainly because it made Bobby a relatively normal, not very funny adult which eliminated like half the comedy from the show.

Aww man, I thought Bobby was great in the reboot.
I haven't tried the new one yet, but I was binging the original as prep for it and hit episodes like Peggy drugging Hank with random doses of oral testosterone without his consent, and it was too much. I felt like the show was very thoughtful about issues like bigotry and child care in the beginning, but it eventually devolved into whatever is funny at the moment without thinking anything through.

Battlestar Galactica. I didn't even really like the show because of the tone anyways... but in the last episode of the first season they throw the female lead in the bed of the male lead who was a "bad guy" for some reason.

I realized they didn't actually care about plot at that point, and were just going for shock value.

Are you referring to when ::: spoiler spoiler Starbuck slept with Gaius? :::
- Yah. I assume they had some trick or something to do with it, but i didn't really care, cause... again, all about shock value.

Two words: Daisy Johnson.

That show was cool, until they're veered off like mad from the OG idea and the MCU, but the same can be said of most Netflix Marvel shows
Jeph Loeb was weird.

I was watching Mr Mercedes and the first season was interesting but after the second show in the second season, I quit. I don’t know what the writers were thinking but that was enough for me.