It is well known that in the 5th and 6th centuries of the Common Era the social complexity and economic productivity of the Roman cultural zone underwent a regression. There were two areas in particular where massive transformation occurred. The interior Balkans and Britain were ethnically and religiously changed in totality from their Roman-era state.
Romanian and Vlach dialects today are a testament to the strength of Latin in the interior Balkans, while Albanian attests to indigenous linguistic diversity. But the dominant languages today are Slavic, due to the putative mass migration of tribes under the leadership of the Avars in the 6th and 7th century. These regions also had to be re-Christianized. Something similar happened in Britain, where mass migrations of Germans transformed the language and religion of the whole region.
But we know that genetically the Balkan Slavs and English are actually genetically more like the earlier populations than descendants of migrants. This is not to say that the exogenous genetic material is trivial. It is significant. There was a mass migration of Slavs and Germans. It was just that these did not contribute to the preponderance of the genes.* And yet the language shifted, and the Christian religion faded.
Late Roman society was defined by specialized economic functions on the production frontier. This was the ancient world’s equivalent of the “just-in-time” economy. In contrast, the Slavic tribes beyond the frontier were arguably even less influenced by Rome than the Germans. These were deeply rustic people. And that was their cultural advantage.
Using a biological analogy, the Late Roman society was like an asexual lineage maximizing short-term gains at the expense of long-term resilience. The “shock” of barbarian incursions in the 5th to 6th centuries totally unraveled the Roman system.
In contrast, small-scaling farming societies organized around clans and tribes, which is how the Slavs were organized, could maintain themselves. Often the Slavs were ruled by non-Slavic groups with origins in the steppes (Avars, Bulgars, etc.), but in the end, the Slavic identity swallowed up their rulers, more or less.
This new setup was successful enough to attract converts from local populations. There is circumstantial evidence that the Anglo-Saxon House of Wessex was actually originally British (look at the early names in the genealogy). They may have been post-Roman British aristocrats who “barbarized.” In Merovingian Francia, Gallo-Roman elites were taking to trousers and aping German Frankish style, but, on the whole the cultural balance was tilted toward the “Romans” rather than the “Germans.” Not so in Britain, it seems.
With all this outlined, it not so surprising that a complex urban society could be culturally assimilated in some ways by a simpler agro-pastoralist lifestyle. The further back you go into the past, the more likely it happened, because the less “robust” the cultural technologies of the urban society were.
* The gene flow into the Balkans was greater proportionally than into Britain. In some cases, more than 50% of the ancestry might be attributable to migrants in the northern and western regions.