The real problem with the NCR isn't its corruption, it's their expansion. Even if they expanded peacefully without stepping over people, they're still going to run into problems in the long run. Remember, the old world ended because it ran out of resources and the population became unsustainable. There are even less resources in the wasteland. If the NCR continues to grow, it'll face the same resource problem that the old world faced, leading to an inevitable collapse.
Caesar's Legion, as evil and ruthless as it may be, is creating a society that can be sustained by the limited resources of the wasteland. The Legion treats everyone like crap but unlike the NCR and the old world, it doesn't depend on limited resources like petroleum and uranium. It won't face a massive deadly collapse like the old world. The worst case scenario for the Legion is that it'll devolve back into warring tribes of savages. The worst case scenario for the NCR is another massive apocalyptic resource war.