The question with those literary sources you mention seems to be this: "How often is genuine depopulation actually intended in these sources? Or how often is it merely a rhetorical device, highlighting a moral point or accentuating perceived military and political decline?"

The most important work on this is Susan Alcock's Graecia Capta (just type Graecia Capta in google and the first hit will be the book - you can read the first chapter for free online). What does she say? Well, in rural areas there was a marked decline, sometimes severe, in habitation of sites. This wasn't just in a few areas either. The starting point however for this was the second half of the third century. There was a lot of movement from countryside to town during this period. This inevitably led to the buying up of numerous tracts of land for large estates and that is the main shift that then took place in the countryside. It started in the Hellenistic era but really took off in the Roman. Literary and archaeological evidence points towards increased nucleation in residences (e.g., villages or other larger rural settlements) where people who did stay and farm "commuted" each day to their land instead of living on it. They did it for various reasons - ease of defending yourself was one of the biggest ones, but also a desire to supplement their income in a particularly difficult time (when more demands through taxation were being placed on them) by living near others and providing other services in addition to farming. All of this led to a decrease in agricultural production though, where some poleis just didn't survive. But when some smaller ones were in trouble, people often left for the bigger towns. Strabo calls Patrai "exceptionally populous", Roman Corinth was a huge and bustling center of the region in terms of political power, population, trade, etc.

So, increased nucleation of population and resources is the biggest single change, but there was clearly population decline too. Greece did become a backwater and museum in the eyes of the Romans, who were focused elsewhere after its capture. For the people in Greece there was change, but they (most inhabitants) would have probably been aware of the nucleation change but wouldn't of thought of themselves as living in a backwater or museum. It didn't start under the Romans though - Hellenistic monarchs represented the same thing to them too: very powerful individuals who pulled together resources from large regions under their control and worked their will upon smaller collections of cities and individuals. They did the same sort of things that the Romans did: bolstered oligarchic regimes, accumulated landed property, realigned territories, and hastened the growth of league institutions.