If either party gains true dominance for any length of time, it will shatter. Both parties are a conflicting mass of interests that are held together as much by the threat of the other party as anything else.
Hard disagree. Republicans vote together on things far more often than democrats do. They have undergone a process of distillation, where their demographics have gotten narrower and narrower, and their positions more extreme. Republicans tend to shift their votes based on political expedience far more than democrats do. Trump insulted Ted Cruz and his wife, and then Cruz kissed his ass. Republicans removed Liz Cheney from her post
in 16 minutes over her saying "the election wasn't rigged" (unfortunately we have no record. We have a recording of Paul Ryan saying "No leaks, that's how we know we're a family". Their recent platform was "whatever Trump wants". And Russia may very well have some kompromat on them. And when the politicians demonstrate they don't care about people dying to covid or global warming, then we know they don't care about policy.
So the politicians don't actually hold any values, and the voting base is ready to shift their opinions to support their politicians, so before any sort of breaking apart happens, we would likely see a repeat of the Nazis. If they crush their enemy that is the democrats, then they will need to find another enemy for their voters to rally against and vote and donate over. Maybe it's immigrants. Maybe it's the homeless. Maybe it's the "deep state communists". And their base is already primed to go after those groups. What issue is going to divide them? Nothing comes to mind. Compare that to the democrats where conservative democrats refuse to accept marijuana or single payer healthcare and progressive democrats cheer those on.
And didn't they have that "any length of time" already? Two years of full control and they worked on stacking the courts.