The Republican party did not collapse and nor is the Democratic party collapsing.

A couple of interesting things happened in the Republican shift in 2016

There was a robust discussion of where the party was, where it was going and what the cadre wanted.

The winner of the primary, instead of shifting left for the general election, actually tacked closer to what the rank and file Republican wanted and has governed accordingly. The end result has been a traditional Republican administration - pro-life, lower taxes, lower regulations, stronger immigration rules enforcement, less pandering to identity groups, stronger defense, a straight forward defense of America's interests over random countries, along with a few traditionally left interests - protectionism, avoiding wars, etc.

This was in contrast with the previous leaders of the Republican part, who tended to govern as per their own interests, after previously campaigning on Republican themes.

The Democratic party is slowly starting to have a similar discussion, although it is not as sharp as the 2016 Republican one.

In any case the Democratic party has the control of most of the media, the house, and its pandering to identity politics means that it will always have a bank of voters who will always support it, irrespective of competence or performance. Also a lot of voters always like free stuff, especially when they are told someone else will pay for it. So for good or bad, the Democratic party is not going away.