We've had that exact scenario happen before, where one party died off entirely. Within a couple of years, they split off again into two different parties. And that's exactly what will happen again. Democrats aren't some kind of monolith, they have everything from Corporate Dems like Pelosi that made millions with her inside trading, don't-rock-the-boat neoliberals that would be considered conservatives anywhere else, to leftists that would love to eat the rich and get money out of politics using just about any means necessary.

They're only grouped up now because the alternative is well... Republicans.

Absolutely disagree. Part of the criticism leveled at the Democratic Party is that it has internal disagreements - they don’t always vote the party line. That means the Republican Party as a single party would be devastating, but Democrats would only loosely keep their hold.

We’ve had far more people in recent elections wanting to vote for a third party but deciding against it because it would waste their vote. With one useless (R) party removed, that just becomes second party.