On the issue of whether Trump is “done,” as I put it below, the primary itself really isn’t the question. There seems little doubt that another Republican could defeat Donald Trump for the nomination, though I’m skeptical of whether that person is Ron DeSantis. The more operative question is what you do with Trump after you beat him. Normally you have a primary battle and one candidate comes out on top. It may be a cakewalk or a brutal slog. But one candidate gets the most delegates and the others fall in behind that candidate.
It’s very difficult, though, to imagine Donald Trump losing a hard fought primary struggle and then just gracefully falling in line as a surrogate for the guy who beat him. In fact, it’s basically impossible to imagine that happening.