I think all of this speculation is very premature.
As I recall, not that long ago, there was a lot of talk about the end of the Democratic party, that it would split into separate left, center left, and labor parties. I remember not too long ago talk of the USA becoming a one party state permanently dominated by the GOP. Now, I hear it about the Dems. Both parties have done time in the wilderness before. The Republicans' last time in the wilderness was shorter than most people realize. Yes, there was the disastrous 1964 defeat, but anyone remember who won the Presidential election in the tumultuous year of 1968? The Democratic disaster in 1980 was much more serious, and their time in the wilderness much longer. I can remember thinking for years that if my father could stick to his GOP loyalties in the long reign of the New Deal, then I could stick this out (and I did). And now, look where we are. The wingnuts may hold the floor now on the right, but they won't last. The party leaders (and most of their members, if not their "base") want to win and return to power. They won't do it with a party made up only of resentful Southern white people and religious fanatics. The Republicans are going through the same internal argument the Democrats went through after 1980 (an argument that still remains unresolved, but "managed"), the argument between party identity and the expediency necessary to win.
I'm not ready to write the Republican obituary, nor would I welcome a one party state dominated by the Democrats any more than I would welcome one party GOP rule. A viable opposition is necessary for a healthy democracy. The Republicans will pull things together eventually, though it may take them a long time. That's fine with me. I just hope they do so AFTER we get universal health care.