It stands to reason that Rome's collapse would have been straight away better for everyone, if it had involved a direct devolution of power to local administrat

It stands to reason that Rome's collapse would have been straight away better for everyone, if it had involved a direct devolution of power to local administration; but that is not what happened. There was a long period of conflict and privation that accompanied it, as all the lights went out on the old administration while people were still running power for the new. To a limited degree, the collapse of the British Empire, gentle though it was, illustrates the same problem: it's not that the new order is worse than the old, but rather that the process of getting there is so disordered and bad.