Here's the thing, what happens to the NCR really depends on what the cannon ending of NV is, if the NCR ending is cannon then I doubt we'll see civil war in the republic any time soon, after all why would we they'll have won the was and with the income their getting from Vegas it should help to fix the economy. Now if the House ending becomes cannon then I think there's a really good chance the NCR will go through a civil war, either that or a all out collapse but since the NCR is the biggest faction post war to date and the most civilized then I really cant see the devs taking them out of the picture.
I agree and would add that civil war is less common in democracies than in dictatorships, which all the other factions are, because there's a legitimate way of getting rid of a bad government. Losing the Mojave would be more likely to result in a change of President than the end of the NCR. It would go back to lick it's wounds in California, protected by mountains, deserts and oceans.
To my mind the FO world is akin to Europe after the fall of the Roman empire, with tribal groupings, some with pre-fall histories, vying for power and establishing nations based on geography.