I’m not a classical historian, but isn’t it already widely known that by the 2nd century, the Preatorian Guard, for example, was mostly made up of non-Romans from Gaul, Germania or North Africa?
I could imagine a case where most Romans themselves were either mired in luxury or poverty, or were so politically factionalized, they were no longer any use in controlling the vast empire.
In short Rome became “Diverse” and then collapsed.