The GOP will never implode though. Not because I think it's some sacred institution or whatever, but because it will be incapable of re-branding itself. The voters themselves will drift away from it (and at a much slower pace, the candidates), and it will fall into obscurity and replaced with something else, like the Democrats. If anything, I'd say the Dems will split off into two parties (like the aforementioned blue dogs and.. blue cats?, and the cycle will continue.
But this would be on the time-scale of another generation or two. Like, "people will have to die of old age and be replaced" kind of scale.