I liked this idea just to imagine the siliness of your existing characters retiring and you have to go fight them with new ones... but its exactly that which makes a D1->D2 repeat unfeasible. People expect to keep growing their existing toons in expansions.
But I imagine the point of the ending is more that, in general, the nephalem are an issue. Without the worldstone there will just be more and more of them getting powerful, and they have evil in their nature so can always be a threat. Therefore the real xp2 bad guy will be a nephalem making a bid to ascend to power, possibly even Kulle (who is notoriously hard to kill, even the crusader comments on it now).
Of course events might still be manipulated that your char ends up taking out a good portion of heaven for some reason.