Something that I find interesting with Rome is that arguably one of the ways it managed to keep going for so long is that it was continuing to push its borders outwards through conquest. Assimilating a land and its people into the Republic/Empire is one way of dealing with the problem of invading “barbarians” (even if that is just transmuting the problem such that your external threat is a new group of “barbarians”, and the old potential invaders potentially pose a threat from within).

Continuing to push outwards is a way to continue developing the military though, and to distract the military from the potential option of seizing power for themselves. There’s only so far you can push before the borders you need to secure are too large to do effectively, and the sheer area to be administrated is too large, even for Rome.

As you highlight, it’s a common misconception that people don’t realise that the Fall of Rome was far more protracted and complex of a process than a single event. I think that’s a shame, because I find it so much more interesting that historians can’t even agree on when the Fall of Rome even was.