I mean, 53 million Americans voted for them (the last time I looked at the numbers), which is no small potatoes. The electoral college numbers tilted pretty heavily Democrat, but the margin of victory was still within a few percentage points in a number of states. I think it's a matter of growing more reasonable about compromise and their social policies, divorcing their policies from overt religion, and then they would grow in mainstream popularity again once people are convinced that the shift is genuine. After all, eventually the Democrats will screw up (some believe they screwed up already this time), and a Republican Party that's come back a bit toward the center could easily take advantage of that the next time it happens.

But that said, a fade to irrelevency is certainly possible if the party continues to push toward the right.