The B.O.M.B. setting the wasteland back would be a fitting conclusion
I disagree with Avellone that the Fallout world had to be reset, civilization would always try to recover in some way or another and bombing the NCR back into the stone age would feel to much like "see the Fallout world will never truly recover", something Bethesda is also kind of doing.
That said, I do think the expansion of the NCR would come to an end somehow, the Colorado river being its frontier with a few sporadic outposts beyond it is IMO an ideal solution. The NCR simply does not have the resources or manpower to expand and colonize all of the South West, it would take decades if not centuries of growth of both population and the means to expand such as infrastructure for the NCR to spread into the rest of North America and I am pretty sure that they would run into other governments who have set up their own nations and established borders around these.
But the few missiles B.O.M.B. would fire (the player could never prevent the launch of all of them) would definitely slow down upcoming nations, and make the Fallout atmosphere last for a while longer.
IMO the best solution to maintain the Fallout atmosphere is not the reset button but to simply set games before Fallout 1, or during Fallout 1 and 2, 2 and New Vegas, and place them in regions far away from the South West.
I strongly feel a sort of deadline needs to be established, sort of a 'after this point the world has grown to civilized and this is no longer like Fallout 1/2', otherwise you end up with a Warhammer 40k scenario; in the future there is always war and its is always five minutes before twelve 'o clock.
Such a deadline does not limit creativity, there is plenty of room in the Fallout world for complete new campaigns, and remember; the BOS, NCR, remnants of the Enclave Super Mutants, Ghouls, do not have to present everywhere for a game to have a Fallout atmosphere. That idea is what Bethesda's writers and marketing department tries to sell you; that these things is what Fallout is all about and only about.