Trump's still got a lock on roughly 50% of the GOP base, and that's not going to really change no matter how much a dumpster fire his campaign becomes. That's not something that the party leadership can just ignore, even if they can somehow kick Trump to the curb and retain the House and Senate. The anger and hatred that's been stoked over the past 8 years especially will still be there and Hillary presidency, even hobbled by a opposition Congress, will keep things nice and hot. And party leadership abandoning Trump won't help things, not when Trump is likely to do everything he can to pin his loss on Democratic cheating and Republican cowardice.
But the biggest blow we're likely to see to the GOP in the short-term is the collapse of Fox News as a PR firm and means of keeping party members on message. I don't really think you can overvalue what they've done to energize the GOP base, promote conservative ideas, and push right-win narratives. When that finally comes crashing down, and I think it will within the next year, the loss is going to be almost immediately felt.
