A generation when speaking of societal changes is pretty insignificant and makes it in no way a stable occurance, especially if you add in beings that live for hundreds of years (ghouls) and have seen things change time and time again.
Its already on the brink of breaking down, you have disenfrachised factions that are pretty large and influential (FoTA is one example) and you have a system that can only exist 'stably' as long as it is expanding (apparently). The first time the NCR meets a force that can defeat it, it would probably shatter. Why is that? With so many different factions unsatisfied within the NCR they seem only to be waiting for an excuse to leave but are unsure of their ability to defeat the NCR as the NCR has, up to this point, been an undefeatable force. If the people could see the NCR be defeated it would encourage them to rise up against it and regain their independence in smaller city-states like Vegas. Of course the same is true with Caesar... there are no real 'essence' differences between Caesar and the Republic their means to an end are different and Caesar is more focused due to a smaller form of tyranny but both suffer the same core problems. The difference is that it would be easier and more likely that Caesar could change his empire to a stable form of governance while the Republic would have difficulties doing so due to the entrenched interests that it builds by its very nature.
Okay. I still don't see the evidence of this happening though. You say that it would be likely that the NCR would be ousted by the native Mojave residents. Thing is, although many Mojave residents express discontent with NCR and state that they would prefer independence,
would they actually fight for it?Let's look faction by faction.
Followers of the Apocalypse aren't going to use military action to oust NCR.
Enclave is utterly destroyed by now.
Raider groups like the Vipers and Jackals are all but obliterated.
Great Khans haven't got a chance; Bitter Springs comes to mind. They're chem dealers and raiders, even the NCR was able to slaughter them.
BoS will hole up in their bunker (again.)
Vegas residents aren't warriors.
The Kings don't seem the type for open warfare.
In order to oust the NCR, you'd need an insurgency. No one faction could go toe-to-toe with the NCR military, nor are many of these groups likely to work together. But enough insurgent attacks could put enough political pressure back in California to "withdraw," so to speak. Now, is taxation enough to make the average citizen an insurgent?
Who'd you be more likely to violently revolt against? Someone who imposed martial law or taxed you, or someone who enslaved your wife and daughter and crucified everyone else you know?
CL is far more oppressive than the NCR, and more likely to face a revolution; although it would
still be unlikely because of CL's ability to impose fear.
So in my opinion, if either NCR or CL take over, they aren't likely to face wide-scale revolution. Pockets of insurgency are far more likely, and I don't believe that would be enough to oust NCR; their assets in the region are too great now; they have the Dam.