Admittedly I am speaking in very broad terms. I disagree strongly that there is not a concept of unified human understanding. Sure it is not a prevalent and direct force, but it is there nonetheless. No nation in Europe (or nowadays the entire world) was/is secluded from the lessons learned from the next nation. Stories, habits and customs bleed over to the surroundings over time. I would say that the Roman experience of Kings under the early city is entirely a different animal compared to the Imperial rule. There are many shades of tyranny, many different combinations of the same thing, one must experiment with them all before moving on to the next step like a baby putting things it finds to its mouth while crawling through the dirt.
Your suggestion that regression is just as likely is only true on the microscopic scale, but on the macroscopic, even a time of troubles is sometimes required to make it to the next level of understanding, and if you look back at the last 5,000 years of history I think that is abundantly clear.