The problem is that people frequently use this type of argument when they are unable to spell or follow the basic rules of syntax and grammar instead of simply admitting they're wrong.
Language does change, over time and across many cultures. It doesn't mean that anything you write is automatically correct.
I've always been a big advocate of the idea that the only part of communication that matters is communication. If people understand you then congrats you've successfully languaged
One thing I learned as an information technology engineer: language is a tool for communication. As long as the sender can send its message unobstructed and as long as the receiver receives and understands the message as intended, the information transmission can be considered a successs.
That said I feel like when people are referring to whether or not something "is a word" they're referring to whether not is has seen historical/widespread usage, not "has somebody ever just decided it meant something, somewhere, at some point"
"a language that doesn't adapt to an ever changing society is bound to be lost", sure, but adapt too quickly and you lose the ability to communicate between groups of people.
There needs to be some compromise where new words are adopted, and changed words are accepted, without flooding the language with garbage. For example, English should still be taught in schools, and English teachers should still have the freedom of correcting the writing kids produce, and taking points off for "mistakes".
Like, if you go pure descriptivist, "it's" and "its" can now mean the same thing. There is no ability to distinguish between their, they're and there. A business email describing a product as "cheugy, no cap" is perfectly acceptable and it's up to the reader to figure it out, because every word is a real word and perfectly valid, and every grammar deviation is acceptable because languages evolve.
Even on social media, I think it's fair to push back on "mistakes" that make it hard to understand something. An error that might take a poster 1 second to fix, might cost the world minutes, as thousands of people each take a few seconds to puzzle out what the OP meant to write.
Languages are about communication, and that can suffer whether the language police are too rigid and forbid any deviation, are too easily bribed and allow for anything.
I'm old enough to have noticed that a huge amount of language has changed in American English in the Westcoast at least. It's pretty remarkable even myself and other middle aged people I know have changed their word use and slang.
"refrigerate" at least has sensible etymological roots in its constituent components.
The problem with brain rot lingo is that it isn't constructed from precedent but a decay therefrom, corrupted by niche "meta" references that are little more than inside jokes that escaped their in-group, divorced of the context that brought them about.
...
Then again, though, the most popular word that humans speak all over the world is "OK", which is itself a memetic corruption of a fad, wherein people were saying "All Correct" with a deliberately exaggerated fake British accent: "Oll Korrect" (which became abbreviated).
And brain rot does have the fact that it's very funny going for it. It sounds silly which makes it fun to say and it pisses people off which makes it even funnier, because getting mad about it is a drastic overreaction. So I don't think it'll even really BECOME an actual serious problem, because the moment it hits mainstream and corporations start publishing commercials about "skibidi Ohio GYATT" it's going to implode like "it's morbin time" burned Sony.
Otherwise, constructing new words out of extant etymological particles is DELIGHTFULLY useful.
In Minecraft, I built an Enfenestrator:
A window through which zombies throw themselves into a catchment chamber for culling and (when zombified villagers are isolated) curing.
When you read texts of an ancient language than span several centuries, and the language itself stays the same, it's a strong indicator the language was no longer spoken.
Living languages always change. Only dead languages stay the same.
English dictionaries are also very much on the descriptive side of things as of late, especially compared to their counterparts among other languages.
Dunno how the tea totallers do things but here in burgerland we actually have sort of a minor annual event finding out the latest slang terms and grammars that have entered this year's edition of the webster dictionary, and which words have fallen out of significant use enough to be dropped from the book too.