hey y'all
I agree with y'all on some of the causes, although my opinion is a little different. My theory is that 4th century Rome was just not ready or willing to deal with the 4th century problems. The lead idea is good, but not the main reason. The mercenary idea, same thing. Same thing for all your ideas, I do say, however, that Rome fell because of ALL those things happening at ONCE. Think about it for a moment: bureaucratic ineffeciency, widespread vice, lack of competition/little frontier space, and other nations rising to prominence. I say the rise of the barbarians and Byzantium ended the Roman epoch and started the European Dark Age and the Byzantine Epoch.
This sort of thing happens for every epoch (the Pelopennesian War ended the Hellenistic period and ushered in the Alexandrian/Macedonian period, the industrial revolution destroyed the socialistic, feudal system of lords and vassals and gave way to a more "federal" system of government). I call the years between the epochs the Transistion Period, and it happens a lot in the world, but it doesn't have to be the collapse of the superpower or the assassination of a world leader.
Think of other times like that in history, the dark ages before the enlightment and age of exploration, America's minor political and economic decays before Jackson and the westward movement, and I agrue the current epoch as well, America's total cultural decay before the next frontier opens up the free-market space race. But then again, I'm an amateur who loves Roman history and this is my my theory on the fall of Rome and other empires throughout history.