The Roman Empire was not economically stable. It was held together by a parasitic military regime, that could only be fed with a permanent influx of plundered riches, enslaved prisoners of wars, and stolen lands.
After the low hanging fruits were reaped, only remained to be conquered poor or remote countries that were not worth the cost of being subjugated — including the cost of maintaining a cohesive Empire over such a large swath of land with the administrative technologies of the time. With the diminishing returns of external plunder, the Establishment had to resort more and more to internal plunder to satisfy its need for riches, resulting in a general impoverishment of the Empire's population and an according shrinking of the Establishment's tax base.
Diocletian's economic totalitarianism, with its combined inflation and price fixing, hereditary jobs and religious tyranny, all imposed by the most righteous of men, indeed a wilful emulator of Cincinnatus, were the death throes of the Roman Empire as a unified central government before it exploded into parts. When intelligent dissent had been quelled for centuries, no wonder our "hero" had only idiotic mass-criminal policies to "Save the Union".
Whereas the conquering Republic could levy an army strong of many hundreds of thousands of citizens from just Rome and its immediate vicinity, the last Emperors of Rome could hardly gather a few tens of thousands of mostly germanic troops in all their Empire to nominally defend their title against foreign invaders. Isn't it remarkable indeed that the more numerous citizens of the richer and more civilized Empire would let themselves conquered by hordes of prehistoric barbarians, and only find other such barbarians to defend them?
Yet, all this shows is that it is easier to find poor men eager to grab their share of loot as victorious conquerors of a rich country, than relatively more comfortable men in said country willing to risk their lives to defend the privileges of a loathed Establishment. There was little popular resistance, both because the people had nothing to resist for, and nothing to resist with. If the central government had ever been good at one thing, it was at violently quenching any attempt of local people to organize in the defense of their rights against or outside its own arbitrary administration. Now that this administration had burnt its fuel, i.e. their lives, the citizens were as unable as unwilling to do more for it. The barbarian invaders might not have been liberators, their yoke wasn't worse than that of the late Roman Empire, and its renewed totalitarian bureaucracy, the Catholic Church, with its anti-economic, anti-materialist ideology of a near but future paradise, right after the coming End of the World. As I wrote earlier, in The Priority of Virtues:
Thus, when the Roman Empire fell to Germanic and Hunnic invaders, it had turned into a totalitarian bureaucracy where even "free" men were burdened by extraordinary taxes and exacting regulations, whereas the Germans and Huns still applied Common Law amongst each other, and were tied by personal bonds rather than impersonal Statutes. Unlike the "citizens" of the Roman Empire, the barbarians still had a Self to fight for and to assert violently. Ignorant and unrefined as they may otherwise have been, the Germans knew some essential principles of Law that had been lost in the Roman Empire. In a very strong and essential sense, they were more civilized than the populations they conquered.
Which only reminds me how the famous Will Durant quote is more current than ever: A great civilization is not conquered from without until it has destroyed itself from within.