I read Gibbons and concluded that mass migration of the barbarians was a major cause of the collapse of Rome, along with corrpution.
It was that, and the fact that the Roman's population declined so much that they didn't have enough men to maintain the borders. And the common man really didn't give a darn by the end.
Funny thing is, it was the "barbarians" that gave Western Rome its final flowering. The Gauls, Goths, and other Romanized tribes were the last line of defense in both the border and the culture war.