True. But look at the political parties that have in fact disappeared: Federalists, Whigs, Know-Nothings, Populists. Two of these were national leaders that elected presidents. So I think that the likelihood of them disappearing is at least as high as them remaining. Besides, we recovered because we are right!
The Republicans are not the party that existed in 1976. Ronald Reagan was not wanted by the party controllers. Republicans, by and large, back then were no more pro-life than the Rats. They made no meaningful moves towards tax reform, and were, for the most part, as pessimistic as the Rats about the future of America. The party insiders hated Reagan. They were more than happy to be a minority in both houses and sit in the country clubs sipping martinis and lamenting the hoi polloi. Reagan turned the Republican party into the party of the tax cutters, the optimists and the conservatives.
For the Rats to survive, they need someone from way outside the system, and not to the left of it, that can pull the party away from the Kerrys, Deans, Clintons and Pelosis. I don't see it happening.