We may not see that today, but that was fairly common for Rome. Marius and Sulla's civil wars, Caesar's civil wars, and Octavian's civil wars all took place in a century. And chalking it all up to "monarchy is unstable" ignores the collapse of the Republic and the 4-15 centuries that the monarchy did pretty well.
As to whether the monarchic system was the reason Rome couldn't keep the whole Empire under one roof -- well, the Byzantines lasted, but they did so while controlling much less territory (Justinian's conquests were quickly lost), so while my argument is weak, I admit, your counterexample is also. The monarchy was successful from Octavian until Commodus; after that, it is hard to name two good rulers who succeeded each other, and impossible to name three. I think it is not by accident that during this early period the Senate was still politically relevant and vaguely democratic. But I am also just strongly tempted to blame regional divisions on poor governance, although there were probably other factors, and I am hardly a scholar.
>I think it is not by accident that during this early period the Senate was still politically relevant and vaguely democratic.
The senate had no actual power starting with Augustus, who turned it into a ceremonial institution. The only time it did have a shred of power was a succession crisis, and it maintained that power throughout.
>But I am also just strongly tempted to blame regional divisions on poor governance, although there were probably other factors,
I don't understand what you're saying here.
>The senate had no actual power starting with Augustus, who turned it into a ceremonial institution.
Early Emperors cared what the Senate thought, even if they didn't have to. Later ones didn't.
https://en.wikipedia.org/wiki/Roman_Senate#Senate_of_the_Rom...
>Around 300 AD, the emperor Diocletian enacted a series of constitutional reforms. In one such reform, he asserted the right of the emperor to take power without the theoretical consent of the senate, thus depriving the senate of its status as the ultimate depository of supreme power. Diocletian's reforms also ended whatever illusion had remained that the senate had independent legislative, judicial, or electoral powers. The senate did, however, retain its legislative powers over public games in Rome, and over the senatorial order.
~ Glen Bowersock
The most important reason for the fall of Rome is frankly obvious, so obvious it's never mentioned: the severe political divisions within the Empire which caused it to fracture. The bulk of the intellectual and economic power in Rome shifted to the Eastern Empire, which lasted in its original form until the eighth century, and in some form until the fifteenth. The seeds of the fall of the Western Roman Empire in the fifth century were sown when Theodosius divided the Empire among his sons in 395, and there is hardly any comparable event today. There were various other reasons, of course; disease certainly didn't help, neither did the appearance of the Sassanids, the increasing irrelevance of the Senate, or famines in Gaul. But everyone on the right and the left wants Rome to have fallen because of its mishandling of their favorite political issue, be it inflation, military conscription, climate change, taxation, or as one amusingly inaccurate meme charges, religious intolerance. These accusations are all silly. The Roman Empire saw multiple civil wars between the reign of Constantine and the conquest by Odoacer. No developed country today faces such an issue. As soon as it was no longer able to draw support from the East, the Western Roman Empire began eighty years of monotonic decline.
If there is any moral to the story, it's that monarchy is unsustainable -- but we all knew that, anyway.