On the rest, yes, I agree with you. Most economists teach a theoretical framework that has been shown to be fundamentally useless. -- James K. Galbraith
The empire can really be seen to crumble in either 337AD with the split into east and west, or 376AD when Valens allowed the Visigoths to settle.
That isn't 4 centuries Life should consist in at least fifty percent pure waste of time, and the rest doing what you please.
This meant they didn't have access to the full Rome-based military and political machine. After the start of empire the emperor had a monopoly on that - which did make it less effective and less flexible.
Effectively if you wanted the top job you got it by taking down the emperor, and not by winning new territory. So the emperor became enemy number one, and energy that could have been spent on expansion was either diverted to internal wars of succession or monopolised by emperors who were trying to maintain their position.
And of course it depends how we define "stable". Civilization of the western Mediterranean changed as much between -100 and 100 as it changed between 300 and 500... Auferre, trucidare, rapere, falsis nominibus imperium; atque, ubi solitudinem faciunt, pacem appellant.
