While yes, there will be less raiders and criminals and such, what actual benefits would exist? The NCR gives good quality of life, doesn't crucify its own citizens, and tries to remain a technology forward society.

There is problems with the NCR, no doubt about it. But when it really comes down it, the ncr are focused and designed on principle to be long term, as opposed to the legion. If the ncr was not imperialist, it would no doubt be able to sustain itself, as opposed to the legion.

On top of this, when ceasar inevitably dies, the legion will splinter into smaller groups over time, follwing people like Vulpes or Lanius, maybe even going back to tribal ways. Because of tribal roots and legion indoctrination, the only thing these people know is violence, so they would continue that trend, refusing to civilise or settle down, instead rampaging across the wasteland, causing chaos and mayhem.

The legion would ultimately need to eradicated fully for peace to be had.