Parties change. What the GOP is today under Trump is not likely to be what it is in even ten years. But that’s only the reality if people who want it to be something different in future stay in it, and work to change it from the inside. Instead, a whole bunch of them are taking a walk, goaded into doing so by people who rabidly disagree with them and know they’ll be in greater control, and more dominant, with the dissenters out in the wilderness, doing none of the things that actually get you power and influence in a political party.
And let’s be clear: These people aren’t likely to find an actual home in the Democratic Party—which is very unlikely to embrace core right-of-center principles anytime soon, though of course these folks could engage with them and try, just as they could with the GOP. They’re also not likely to stand up a third party that can prove electorally competitive or functional anytime soon. If we’re doing a simple mathematical equation assessing where one can most efficiently effect change, the answer still remains the GOP—even if the odds look really slim.