Again, deep diving into the lore, the NCR was on the decline already. We see it in New Vegas. The Bear was bloated and encumbered itself with pointless wars of expansion and major corruption. The Boneyard Seperatists were gaining traction, and other parts of the country were in economic distress. The Mojave Territory is simultaneously over and underfunded, and is being used as a distraction from the major internal problems. If Shady Sands was nuked in a non-NCR ending to New Vegas I could see every city rising up and forming their own states.