You can't use the end of the Julio-Claudian dynasty's end as a POD because the Senate had long sense lost any semblance of control over the Army and the Generals; the moment they end the Principate the various armies will march on Rome, each led by a would-be Emperor.
I mostly agree with you on this. I'm not 100% convinced some kind of rollback of Imperial authority would have been impossible, but it's a very, very difficult needle to thread.
The Late Republic had broken down for a number of reasons, but one of the biggest was that the size of Roman-dominated territory and the scale of ongoing military activity required long-serving professional soldiers (mostly recruited from the lower socioeconomic classes) under the command of long-serving Proconsuls (and other promagistrates). And under the circumstances, and given how central Patron/Client relationships were to Roman society, it was very hard to keep the soldiers from seeing themselves as clients of the Proconsuls they served under. Likewise, the revenues collected from the provinces and loot collected from the wars flowed through the hands of the Proconsuls, giving them control of considerable resources they could channel in ways that strengthened their individual client networks. The end result was that individual leaders (most notably Julius Caesar) could build up client networks to rival the power of the Senate as a whole.
Octavian's solution to this was to assign himself (and his heirs)
direct personal ownership of many of the Provinces, particularly the frontiers that required large garrisons (thus putting most of the army in his provinces) and the recently-acquired big revenue-generating provinces of the far East (giving him plenty of cash flow to pay his soldiers and other clients). These provinces were governed by Legates who served at the Emperor's pleasure, not by Proconsuls who served for long, fixed terms. And it was structured in such a way that the Legates were the Emperor's clients, not independent power centers in their own right who could build up their own client networks.
Any attempt to replace the Principate with something more like the Late Republic would have to find another way to avoid the problems that lead to the
de facto end of the Republic in the first place. Simply converting the Imperial Provinces back into Senatorial Provinces would run into the same problems. I'm not sure the best way to do it, but two families of possible solutions occur to me:
- Tighter control over the provinces, limiting the ability of provincial governors (be they Legates or Proconsuls) to operate unchecked in their provinces. Allowing the Senate to replace governors at will (as the Emperor could do for his Legates) would be one possible measure. Another would be to extend the principle of plurality of offices to the provinces, so each province would be lead by two Proconsuls with mutual veto power, not by just one as in the Late Republic. The risks here are a) it doesn't go far enough and another Caesar arises, or b) the checks and balances undermine the ability of governors to control their provinces and effectively counter military threats on the frontier.
- Retain the principle of personal ownership of the Imperial provinces, but divide them between a large number of families so no one family can accumulate too much power. Octavian, Anthony, and Lepidus had tried a variation of this for the Second Triumvirate (a three-way split of the provinces), which obviously didn't work, but a 20-ish-way split might work better. Or it might lead to stronger power base for the 20-ish Permanent Governors, leading to them fighting among one another for control, or to increasingly-smaller groups of them banding together to take control of the state away from the Senate and the other Permanent Governors. Or splitting up the provinces finely enough to minimize the risk of a civil war might prevent any one frontier province from being strong enough to defend itself from external threats.
And then there's the problem of how to get from A to B: at the end of the Julio-Claudian dynasty, most of the frontier Legions have thought of themselves as clients of the Caesars for the better part of a century. They're going to provide a ready-to-hand power base for anyone who can plausibly claim to be the proper heir of the Caesars, just as Emperor Constantine described above.