Greece wasn’t a state back then. The only time there was a unified strictly Hellenistic state was under Alexander the Great, so I wouldn’t say the Greeks didn’t thrive at one point. It’s just natural for empires to fall. The purpose of an empire may appear initially to be to control many people and lots of land, but historically it has been how scientific progress and philosophy has spread. Now that we live in a post imperial age, this is no longer the case. Hence why I believe there are a few countries destined to fall apart: US, China, and Russia. The imperialist model is outdated and each country will destroy the other from within by exploiting cultural differences and exacerbating social issues to cause chaos.
I’m rambling at this point but the first half of this comment holds.