By the time Constantine began his reign, the Roman military could no longer sustain itself and was forced to hire mercenaries to defend its borders. This contributed to a further weakening of an economy crippled by civil war, and forced the Emperor to institute drastic economic reforms that only delayed the inevitable end. With the confederation of many of the barbarian tribes, and given its dwindling resources, Western Rome could no longer protect its borders. The end was assured by the start of the Fourth Century, although the Empire struggled on after that, even managing some semblance of its former glory whenever it could stop bickering long enough to raise an effective army.
While it is simplistic to attribute the fall of Rome to moral decay, it is simply revisionist to pretend that it didn't play a role. Constantine's reign marked the last flare of the Roman candle before the flame was extinguished permanently.
You have done an admirable job of summarizing the military and economic factors leading to the Empire's decline after what Rostovtzeff termed the "Crisis of the Third Century", but you haven't given any indication of what part you believe moral decay played in the collapse of the Western Empire. As I pointed out, many contemporary Romans considered the spread of Christianity to be "moral decay", for the reasons which Gibbon noted - detachment from secular affairs, monasticism, etc.