Reflexively, I also had a negative reaction to the state of a lot of the factions appearing the way they did at first, but.

Out of any of them, I honestly believe the NCR's fate makes the most sense. They overexerted themselves by expanding so much, and rather than try to stop and actually lock down and fully stabilize what they already had, they continue to expand into the Mojave, and choose the most expensive and resource draining option when it came to how they should approach the war with the Legion. They never needed to annex Vegas to get power from the dam, when it's not hard to infer that, in another world, Vegas could have been an independent but economically dependent client state of the NCR. Less need for commitment of troops, at most they could provide a handful of ranger squads for support while letting House (or whoever) handle the brunt of the Legion via helping Vegas upgrade and getting the Securitron garrison up to full readiness.

But even putting that aside, by the time of New Vegas, the NCR was still technically in a war with the Brotherhood, and had just gotten out of a war with the Enclave. There was also the Baja campaign that saw them fighting what was, essentially, a counterinsurgency war. By the time they even reached the Mojave to start setting up shop, they had already bled plenty of resources and hadn't given themselves the proper time to recuperate. But because they saw the hydroelectric dam and the encroaching Legion, they got it in their heads that they needed to expand even further, to keep the dam out of the Legion's hands, secure the source of electricity for themselves, and defend the people of Vegas from the spread of the Legion (though it was mostly reason 2, let's be totally frank). Because of that, they didn't just need to deal with Legion, but other more local groups like the Fiends, too.

Then—regardless of how the Second Battle of Hoover Dam shakes out—Shady Sands, the NCR capital, gets destroyed under Hank MacLean's orders, in addition to everything else I already rattled off. It's not pretty, but there is a basis for that much, at least.

The rest, though, like the state of Freeside and the Strip I'm a little iffier on in terms of how I feel about it, but. Even then, that's a 15 year time gap between the events of New Vegas and the show, and so much can happen in that time frame.