A major problem here is that neither the democrats nor the republicans have ever been good at…

A major problem here is that neither the democrats nor the republicans have ever been good at representing anyone other than a handful of centrists.

In the event of a collapse of the GOP, we might see the dems move further to the right (as they have been for decades) and, say, the DSA become the center-left party -- and for a while that might be a stable situation -- but the particular dynamics of elections in the united states make it difficult for there to be more than two parties (and incentivize the two largest parties to always become mostly-overlapping vaguely center-right parties).

We're seeing ranked choice voting & other similar formats popping up in various localities, and if more widely adopted, these styles of voting may make a healthy multi-party ecosystem possible in the united states, but until then we will continue to have functionally a single-party state where that party has been artificially and arbitrarily split in two (with every elected official being either a republican with an 'R' after their name or a republican with a 'D' after their name).

Of course, representative democracy (particularly the kind with elections rather than, say, sortition) is prone to collapsing into internal corruption and infighting (particularly a combination of public infighting and private collusion, since power is mostly a function of PR in such a system). The best we can hope for without dramatic structural change is to mimic one of the slightly more functional european countries.