Unfortunately the probability of an endless string of good rulers is not so good, the prospect of good rulers containing overmighty subjects is even less likely (see: Justinian and his issues with Belisarius as one example), and the danger of a weak ruler *and* an overmighty subject is how one can wind up in deep trouble. In an autocracy formed in an already-militarized society like the Empire, it only takes one such incident to create the OTL pattern.
The only ways to alter it are to make the entire Megastate system akin to the East-Roman system of dynastic rulers with a bureaucracy that endures no matter who rules, and that was not something the men that would found the Empire would want: like all people who commanded arbitrary, autocratic power they did not voluntarily want to yield any of it.
That's the rub. Although I would note that the concept of succession-by-blood does work, that just limits the pool of potential usurpers, it doesn't translate into "no would be usurpers" any more than Medieval England or the Ottomans (as the longest lasting dynasty that had to wrestle with the "But which descendant of the Great One?" problem) could eliminate the problem.
And I'm not sure how you make that work so as to eliminate the overmighty subject problem when Imperial authority really is arbitrary - meaning that if you win, you are legitimized.
"Treason doth never prosper, what's the reason? For if it succeed, none dare it call it treason."