Immigration and lack of assimilation were the big problems. Rome lost battles, sure, but the Roman Empire wasn’t killed on the battlefield. The culture just faded and it no longer had cohesion. Died with more of a whimper than a bang.
Sounds familiar.
It became “Balkanized” so badly, that it soon didn’t exist.
It wasn’t lack of assimilation. The Frank’s became the French, the visigoths became Spanish and the various Germanis were highly influenced by Latin culture.
The collapse was economic primarily and secondarily due to plagues spreading as so many were living in close proximity to each other, in squalid conditions