The empire's biggest problem was that it never definitively settled the issue of succession.
All emperors were either usurpers or they had acquired their title to the throne by descent from or adoption by a usurper.
Thus any successful soldier had inherently as good a title as any other, and for that matter as good a potential title as the guy on the throne.
This led emperors to, understandably, be nervous about successful soldiers and kill them on a regular basis. Which led some good generals to revolt in self-defense who might otherwise have been perfectly content to remain loyal.
Contrast this with the French or Ottoman empires, where no general, no matter how successful, had any hope of gaining the throne by revolt. Thus he could remain loyal without worry of being executed for winning battles.
Hereditary succession to absolute power has its own problems, but probably fewer than the constant succession and civil wars of the Romans and Byzantines.
Gee whiz! It sounds almost like today where the populace and the judiciary are so ill educated that they cannot define "natural born citizen" and enforce the principle.
And the donatives (i.e., bribery) given by the emperors to the soldiers was a factor that aggravated the instability. Often the soldiers supported whomever offered them the most money.
About to be beheaded on the deck of a warship in the Golden Horn, the Emperor Maurice turned towards his successor Phocas and uttered his last words: “Don’t forget to pay the army.”
Either way the actions would have been the same; power struggles, oppression through "compassion", welfare to appease the masses, over expansion through the ages, massive debt, inflationary practices especially by the “Dominus et deus” I guess the Nixon of Emperors with his price controls, confusing/repressive taxation polices which were different from province to province, “hospitable” immigration "appeasement" (In the West), messing with the currency, depopulation of actual citizens, etc...
In the end once the individual is detroyed by government policies (Some by "suicide" who are granted some type of control over government) and becomes apathetic, "Empires" die.
“the Roman people, were dissolved into the common mass of mankind, and confounded with the millions of servile provincials, who had received the name, without adopting the spirit, of Romans”
This was the root cause of the decline.