TL;DR
Rome didn’t fall because of barbarians, but because of government interference. There were price controls on food, which made food scarce and that caused people to move out of the cities, destroying the manufacturing capacity of cities, which in turn destroyed the food production in the countryside. All of this ultimately was the result of currency debasement and the policies that were enacted to combat the consequences of that debasement. It’s a great read on the second and third order effects of theft by debasement of money.