I am thinking of a very different book here.
The Fall of Rome, Ward-Perkins
A deep dive into the archeological record, potsherds and middens, postholes and roof slates, that sort of thing.
The collapse of material culture, and broadly speaking, its utter disappearance, point to a massive population collapse across Western Europe. Given what seems to have been a cataclysmic human disaster, its certainly wasnt just a matter of switching tax collectors.
I have read Ward-Perkins and just pulled it down from my shelf today for another read (thanks to you! :) )