A LOT of false statements about our actual past here, so far. And a lot of mistaken understandings about what happens when political parties do cease to function well.

The only time there has been a single political party much of anywhere (where there is democracy), for any length of time, is when it was a fake. Such as when the only political party allowed to exist, was the Nazi party in Germany, or the Communist party in the USSR.

What has ALWAYS happened when one large party disintegrated in the US, is that one or more parties representing the fractured pieces, formed a new party. Usually most of the people in the old party changed and joined an existing one, OTHER than their main opponent.

So if the Republicans WERE to entirely implode, which I think is unlikely, it wouldn't leave JUST the Democrats. Not even JUST the Democrats plus all the other tiny parties who most people forget really do exist (https://en.wikipedia.org/wiki/List_of_political_parties_in_the_United_States).