The more primitive the society, the healthier skeletons you will find. Because in a primitive society only the healthiest and the fittest can survive. Whereas an advanced society can take care of the weak and the sick.

That's interesting. But do you have some data to back up your claim or is this just your guess? Because there are two effects at play here: on one hand a kind of a survivorship bias that you mention and on the other hand a positive effect of a healthier environment on a human body. These go against each other and I don't find it obvious to see which one is stronger.

After a quick search I would say the latter is more important here: what we observe from the bones are things like a quality of nutrition (e.g. vitamin deficiencies) or effects of some illnesses. Inadequate nutrition signals poor living condition and illnesses affect even the most "gifted" individuals (don't forget that hardship selects also for other traits than just a good immunity).

I'd wager that if the modern globalized economy were destroyed, people everywhere would become healthier, going from single-crop intensive farming, garment working and assembly lines back to localized farming and manufacturing. The populations would dwindle, of course, but the survivors would be healthier and happier.

It was always my impression that the term “dark ages” was never about quality of life, but created in opposition to the enlightenment (partly because we don’t know much about that age, partly because of the role superstition and belief held in documented cases).

In Austria and Germany most people descripe the Nazi era also as “dark times”, despite there beeing roads built (although the Nazis were so in debt at the beginning of the war already that you can hardly call this a success).

So dark has multiple definitions. I thought the most.common one was about a time where rational thought and humanism didn’t really win, but tribal rivalry (on any scale) and radical beliefs did

The concept of the Dark Age was, indeed, invented in the Enlightenment, and contemporaneous rhetoric did use it to portray the Enlightenment project in a favorable light as a break from a benighted past.

The issue is that the Enlightenment wasn't a project to move human beings to some better plane, but to rationalize society and its members in a way that made it more amendable to control and planning. And so centralized power could use Enlightenment ideology to eliminate alternative power centers, centralizing power in the hands of a single state that could reorganize society for its own ends.

That itself had pros and cons, with the biggest con being the newfound ability for states to execute projects of mass violence more effectively.