I'm quite frankly sick and tired of discussing politics in detail.
And it's a free forum, if I want to just give my quick 2 cents I'm allowed to, even if they are "flawed".
But I can spare a quick argument.
I don't see NCR being able to pull themself out of the ditch they've been digging because of the way they've been doing things.
They've made enemies in a lot of places, even within their own government.
They are corrupt and inefficient to the point of being incompetent at times.
So I see rebellions coming.
The enemies they've made during this, Great Khans and BOS are just two, will take that time to attack and try to destroy NCR too.
I simply don't see how NCR is gonna be able to keep going.
If they won New Vegas they might be able to stabilize things, show their people that they could win and that they are necessary.
But if they fail? Then the people will throw a fit, so many lives, so many resources, so much money, completely wasted into the Mojave.
People will revolt.
NCR will lose lands.
They might not crumble all together, but they will lose a lot.
What army they have left will have to prioritize areas that are prosperous.
Everything else is left to fend for the wolves.
With the army and police withdrawn from a lot of towns, cities and communities gangs will move in, take control, build power and raid NCR caravans.
I simply see downspiral for NCR if they lose.
They can go back and secure their borders?
Pfft, with what? The remnants of their army?
They have to secure their lands from riots and rebellions too.
All speculation though.
Take it or leave it.
But NCR losing would be a downward spiral which is going to hurt NCR lands a lot.
That's my speculation on 'em.
Seen enough characters in New Vegas who are sick and tired of NCR to come to that conclusion.
Seen enough corruption and incompetence.
Seen enough idiotic prioritizing made by the leaders.
And add in that House has been bleeding them dry, and will if they ever make contact with him again (peaceful contact) and it's hardly helping.
And again, House doesn't care.
He doesn't give a rats ass about NCR or Legion.
Legion are savages, he doesn't care.
NCR is profitable? Sure, until he doesn't want them there anymore, then he couldn't care less what happens to them as long as they keep out of Vegas.
Bleh, that's my speculation on it from my experience with the game.
Take it or leave it, still gonna make oversimplifications in the future.
I understand this is merely your speculation, but I feel that many of these arguments are on shaky ground.
Considering the NCR has been attacked by nearly every faction you just named and more, how exactly is it their fault the Khans decided to muder and pillage NCR cities, or the Enclave attempted to eradicate them and the entire continent, or the BoS decided to 'take the technology from the wastelanders.' The NCR didn't make them into enemies, all of those factions made their choice, and they clearly made the wrong one. The Khans are on the run, the Enclave has been shattered, and the BoS is on a steep decline.
By the way, you do realize that in terms of priority, the Mojave is a distant third amongst the NCR's priorities from what we know, as there is still a low-intensity war with the Brotherhood, a much more powerful opponent than the Legion, and there is evidence that meanwhile they are still expanding South (Baja) and North (They're already in Southern Oregon, i.e Klamath), from Hanlons story about the settlers around the well in Baja or the influx of NCR settlers swarming into the Mojave, and without the resistance they're facing in NV, all while trying to keep the Legion out the Mojave. As regrettable (as well as unlikely) as the loss of the Mojave might be, it would hardly be the existential threat you deem it to be, particularly in a House or Independant ending, when they have a convenient buffer between them.
Clearly you underestimate the power of political enfranchisment that representive governments bring. When the population has a stake in the continued existance fo the government, as well as the right to elect their own officials, the possiblity of revolution goes way, way down. And what, pray tell, would be the catalyst for a major uprising? What existential threat to the livelihoods of significant portion of the populace akin to say the abolition of slavery (which the Southern economies depended on) exists in the NCR? The vast majority of the populace is not going to rally around overthrowing a democratically-elected government when there is no guarantee the alternative will be any better, and it is almost certain that it will be worse. Why should they? Not to mention that unlike the real-life USA, the NCR does not have a weak federal government and small (back then, the US had the smallest standing army in the industrial world) military.
The NCR has endured much, much worse than anything the Legion is capable of, fought much tougher opponents than them, and are still standing. In fact, they've prospered. In fact, the Core Region so safe and boring that people like Henry Jamison have come out East looking for adventure. And besides, the NCR is downright angelic in terms of corruption compared to the real-life 19th Century US they're based off of, so frankly, it looks to me like you're making mountains out of molehills. Last time I checked, NCR troops and weren't firing on strikers or busting unions.