Well, by what metric do you measure things?

Let's take: That the 5th century is demonstrably way worse than the 15th century, on average, for all humans. The 15th Century was a pretty bloody century and one where the Bubonic Plague was still recurring. How as the 5th Century worse for all people?

Exactly what metric is that being measured? What is the basis for the conclusion?

It's not a denial of the linear and progressive meta-narrative. It's a denial that any such narrative is proven. The proponents have not made their case. For every area experiencing turmoil, there were other areas showing growth. Empires falling often allow areas that were suppressed to bloom.

Even the 20th Century is noted as much for the amount of people have died in wars and genocide as for its advancements in science.

The meta-narrative relies on the use of the word "all" and the one thing history shows is what "all" is never all or the anywhere near the whole story. There are far more exceptions and exclusions than inclusions that would allow for trends like this meta-narrative to occur. Fundamentally, people are just people and things get very messy down in the details.

Fundamentally, it boils down to the word "narrative". That's the correct word for this. It's a story. It's not history.

I think the issue is that you a sort of misunderstanding what it means for historians to reject a linear narrative of history. I don't think that there is a professional historian alive that would suggest that life was better for Europeans in the 17th century than life for Europeans in the 20th century, for example. What the rejection of a linear narrative of history entails is the acceptance that we cannot put historical changes in a neat, predictable narrative.

For example, consider the spread of technology. Technological advancements are do not spread "evenly" and are not accepted uniformly. Consider the agricultural revolution of the thirteenth century. Traditionally it was assumed that peasants in the medieval west all gradually adopted the advancements of the horse collar, water/wind mills, three field rotation, etc. But as Robert Lopez points out in his book *The Commercial Revolution of the Middle Ages*, pictorial evidence demonstrates that as late as the thirteenth century, or even later, these advancements had not been adopted uniformly. What is more, peasant communities might adopt one or two of these advancements and not others.

Furthermore, older technologies take centuries to be replaced. I have two examples of this. First, the evolution and emergence of print technology. Today, we accept the importance of print and its contribution to early modern culture. But it took centuries for printed works to supplant manuscripts. Manuscripts had a cache, aesthetic, and, perhaps most importantly, tradition that outstripped print. Manuscript culture and print culture existed side by side for centuries, as Andrew Pettegree points out.

My second example is more broad. The archeological schema of the evolution of the stone age to the bronze age and into the iron age papers over a much more complex history. (And i recognize that most archeologists recognize these errors but still use this flawed chronology out of convenience...and other reasons I won't get into here.) While humans learned to forge and work iron, this doesn't change the fact that bronze was not supplanted--despite the fact that iron is much more widely available and often easier to extract. But bronze, once you get the tin and copper together, is much easier to work, and so many groups favored bronze work for these reasons.

Even where these advancements happen, the cultures that adopt them are not always on an upward trajectory, and "technological advancements" did not always, at least initially, lead to a better quality of life. For example, the medical revolution of the early modern period that you reference did eventually lead to improved medical knowledge, but it also destroyed the careers and experiences of what one might call "healing women" or "folk healers." These medical practitioners had a wealth of experiential knowledge that was more effective and practical than many of the leading medical "doctors" of the 16th century, who often relied heavily on classical sources that were flat-out incorrect; they could do more harm than good.

As another example of this, as humans started to adopt settled agriculture, it is possible that the quality of life and expectancy went down because humanity went from a varied diet of meats, fruits, and other products to a cereal monoculture. This happened again during the transition from the early Middle Ages (your awful 5th century) into the high Middle Ages, when Western Europe once again began to adopt a cereal monoculture, as Robert Bartlett describes in *The MAking of Europe*. Furthermore, this reliance on on single crops could also lead to famine, as the crops could all die from a single disease.

Finally, as you point out in your response to u/shemanese 's post, laws become "better"--a quality you neither define nor describe--but you ignore the fact that these laws could be more controlling; taxes can become more burdensome; and the general "freedom" of the peasant class can become more restricted. I would honestly suggest that for vast swaths of the modern period, the lives of peasants was better in the thirteenth century. Indeed, these people's lives could hardly be said to have changed significantly during this period, except landlords became even more powerful.

As a last point, I would like to underline another point u/shemanese makes below about the life expectancy of Indian colonial subjects of the British Empire. I would hardly suggest that life is worse for many modern Indians in the post-colonial era than it was during the Maurya Empire, for example, but accepting that "things constantly get better" ignores the horrors of colonialism, which undoubtedly left a stain wherever it went and made life demonstrably worse. Not only in terms of life expectancy, but, at least in terms of India, literacy declined and the caste system became far more rigid--a development that continues to have consequences today.