True, but what have the NCR and House brought to the table?
The NCR is just an attempt at re-establishing the system that led to the fall, they did well for a while but they've stretched themselves thin trying to play world police while hindered by mountains of bureaucracy, they're attempts at recreating America will inevitably fail when they push out so far that their fragile network of taxes and local governance can't be held together because of just how many tribes rise around and within them.
House is stagnate like I've said, he's hellbent on just keeping his tiny little corner of the world a nostalgic photo of the past, he has a powerful military force and under him Vegas would absolutely be an economic powerhouse but how long can a single city survive when all around him Empires rise and fall at alarming rates? If House never tries to become one with the New world and just keep his tiny little kingdom a snapshot of a dead past it will decay and the living wasteland will swallow him.
An independent Vegas isn't eternal, the Couriers Vegas WILL die, but the fact that it and the players in it were given the opportunity to change with the world around them will at least create the potential for them to survive the ever changing wasteland, and maybe even create something stronger than the NCR, Legion, or House.
Ultimately ALL the factions are doomed to fall, like many before and after them, the wasteland is a living thing, but out of even the smallest things something truly great might be able to rise.