Historians never talk about the "good old days".

Only people that didn't live the old days think those were the "good" ones.

Anyone with a baseline education in history knows "good old days" is proponganda used by almost every bad actor that ever existed

Historians don’t talk about “good” or “bad” unless there’s some unambiguous metric within the historical context.

Santa should publish previous years’ nice and naughty lists for historical reference
- Nah—see Goodhart’s law (“When a measure becomes a target, it ceases to be a good measure”).
  As soon as Santa published his lists, people would start figuring out ways to game it.

I don't know anyone who talks about the good old days.

My great grandparents didn't. My 90 year old parents don't.

They've all seen some shit us "youngin's" simply can't fathom.

I had to read Studs Terkel's "Hard Times" before I had a clue how shockingly hard life was before the mid-20th century, and how easy us born after that have it.

Broadly speaking this is probably true. In a smaller context, though, there are tons of counter examples. The internet for example, from just 10 years ago, was unquestionably better. AI slop, bots, enshitification, social media and browser monoculture...
The anti science trend of MAGA over the last few years...
Etc. Regression does happen, and we should not take things for granted.
- Internet was different, not better. Don't you remember the browsers with 30 "search bars"? The viruses infecting computers by just connecting to internet? The slow speeds?

Until you hear them talk about golden eras.

But it's always about something specific, they know too much to ever say an "era" was objectively good.
I'm Dutch, so we are tough about the Dutch golden age, which spans about the seventeenth century. Trade science and art were at a high point back then, but looking deeper it turns out the life expectancy for common people was lower than in the surrounding countries. The wealth was all concentrated in the Holland province(s), which explains why the Netherlands is commonly referred as Holland in other countries. Another thing was the schism between catholics and Calvinists, which meant Catholic cities didn't benefit at all from the golden age. It also wasn't benificial to the 1.7 million people who were enslaved by the Dutch.
So all in all, the Dutch golden age was very much golden for the merchants and aristocrats of cities like Amsterdam, Rotterdam, Leiden or Haarlem, and for the protestant ministers. For the rest, of you didn't live near these cities you probably were dirt poor.
- yeah "golden age" is similar to "great empire" - "great" maybe for the emperor but for basically nobody else.

It's because they're way more aware of the drawbacks of certain eras.
Slavery, racism, inequality, lack of resources, lack of education, lack of clean water, how many of your children will make it to adulthood, famine, floods, lack of roads...
Every "good old day" was worse in some aspects.

Because they are scientists

Well, no, they're historians. You don't need to be a scientist to prize evidence based thinking but you do need to be investigating natural phenomena through a system based on scientific methodology to be a scientist.
You could stretch that out and point out that everything is natural but then you're dealing with anthropology which is different than being a historian because they use different means of determining fact.
- This may be a language barrier. In my native language a historian is a scientist researching history. But maybe I just don't understand what you want to say.