“The Fall of Rome and The End of Civilization”, Ward-Perkins, makes a compelling argument against this. Through an analysis of archaeological remains, including seemingly humble things like potsherds and rooftiles, he shows that there was a profound disjunct in quantity and quality in every sort of material element of human existence, timed to, almost precisely, the Germanic invasions. This was not just vis-a-vis the height of Roman culture, but also vs pre-Roman times. There was obviously a horrifying catastrophe in Western Europe that sent everything almost back to the Neolithic.
That’s a good book.
But look at the time-periods he compares - from 300 AD to 700 AD — the latter is AFTER the Byzantine-Sassanid war and the Islamic conquests.
If one looks at between 400 AD and 450 AD, there is no material change.
Between 450 and 500 there is a gradual decline, but not as abrupt.
It’s only when one compares across 400 years that the change is startling