Yea, but, No.

There seem to be lots of discussion on how this is the end of the GOP, or the conservative movement or something.  Lots of Dem (leaning, voting or registered) individuals seem to see this and there is an undercurrent of either "Dems will take over" or that lots of disaffected former GOPers will come into the Dem fold or something along those lines, but it won't happen and the GOP will continue for quite a while.  It probably won't even be appreciably diminished.

The thing is that Republicans control lots of state legislatures and governorships, they control the US House and Senate (and thanks to gerrymandering even under the most favorable-for-Dems outcome this election, they will continue to hold the House).  Even if Trump goes down in flames the GOP will keep on keeping on.  Yea, maybe some of their voters will give up but the fact is that the GOP has engineered the system to do well even when they lose badly at the polls.

Remember when they all thought Obama was doomed in 2012, but then he won soundly and the GOP had to go seriously rethink their message (but not their policies)?  Yea, now we have Trump.  It's true that the Repubs would likely be looking at a presidental win against Hillary this fall if they had nominated pretty much anyone other than Trump (or Cruz).  But there won't likely be much rethinking becasue, even if they lose badly, it won't actually hurt them.  They seem to do better when Dems hold the White House than when they do anyway, and Hillary will be a major source of artificial scandal and money for the party for her entire tenure as president.

I think a Trump win would be horrible, but I don't think his loss will do anything to change the Republican party.  The next 8 years are likely to look a lot like the last 8.  Democrats are on a slow roll forward, and 8 years from now--after the next census and redistricting, when we will have an even larger minority fraction of the population and millennials are a bit older--there will be a chance for a meaningful realignment to happen, but Democrats will have to seriously participate in all the elections between then and now, not just this year and 2020.