Funny, I actually voted for economic decline as the single biggest contributor too, but I'd add that the declining economy wasn't some irreducible primary factor, but a consequence of (inter alia) bad domestic policy (e.g., the Edict on Prices, financing spectacles and public works through debasing the currency), bad foreign policy (esp paying more to bribe the barbarians than to build bigger armies and fortifications), Christianity (in the form of spending wildly on economically worthless churches and purely parasitical monasteries), and civil war. In my view, perpetual economic decline was the thing that made all the rest a real danger instead of a mere nuisance.
Funny, I don't see Caesar in your post or anything concerning government.