This feels like a stable equilibrium problem. To get out of the U that they find themselves in, some collective amount of energy, applied at the same time, is required. Individual defections or other actions wont work, and attempts at collective action so far have not succeeded (eg invoking the 25th amendment in the last administration was floated but could not get very far). Barring a truly catastrophic shock that breaks the hold of the supporters, such as a mass shooting/massacre in the Senate carried out by an obviously and undeniably pro Trump person, or an absolutely crushing democratic landslide, I cant see it changing any time soon.
Are the last rational Republicans even Republicans any longer? Why would they want to be associated with a gang of Christian supremacists, white supremacists, neo-Nazis, and professional Know Nothings?