The Roman empire collapsed for demographic reasons. Too few Romans, especially those willing to defend the Empire. And, on the other side, too many barbarians who got ever more aggressive as Rome weakened. At first, Rome was able to co-opt the barbarian tribes. Starting with the Vandals, they eventually figured out that Rome was a paper tiger and started chipping away at the Empire. Rome was sacked a number of time, starting in the early 400s.
I see the same thing happening here. We’ve discarded the melting pot for multiculturalism. To our elites, Diversity = Strength, but in the end it will tear us apart.
I have never seen any evidence the later empire was less populous than in the time of Augustus or Marcus Aurelius. Or that the “barbarian” tribes across the borders were any more numerous.
For a host of reasons there were fewer men willing or able to fight to defend the empire. In fact, there are historians who claim that the “fall of the empire” was as much a series of revolutions by the inhabitants as it was invasion by outsiders.
In any case, it is obvious there were few willing to fight for what had become an oppressive system. Most of them discovered the hard way that life became much harder for them after the empire fell, but by then it was too late.