My understanding was that in the Roman-ruled colonies, such as Britain, regulations became so restrictive, so micromanaging and inflation so high that it crumbled as people picked up and moved in large migrations.
The events described here pertain to the very early empire.
The people picking up an moving were entering the Empire, not the other way. Rome’s implosion in the west resembles the implosion of the post-Roman petty kingdoms in Britain in the face of the Angles, Saxons, and Jutes’ settlement expansion. The Britons raided and burned each others’ towns and territories when the victims were defending themselves against the foreign invader; the Romans were engaged in infighting, intrigues, assassinations, and rival “emperors” instead of managing imperial affairs.