Ummmmm.....ever noticed that Maine and Texas are...pretty different?
You can flip Maine from RINO to Democrat. That's it. It's not possible to elect a senatorial candidate in states like Maine, Delaware, etc. that would meet the purity standards of FR.
Of course, if your goal is to convert the party to a purely regional party of the South and Mountain West, with no chance of winning the Presidency, then that's fine.
Yeah, Maine is a problem, and your arguments have some merit. Suppose Collins decides to run and there’s a conservative primary challenger. The conservative might well be, as you point out, less likely to win the general. On the other hand, there’ve been upsets before. The question is how big of a risk would losing Collins be. She caucuses with the Republicans (at least for now), which affects committee chairships and the like, but she’s otherwise horrible.
Traditionally, in the midterms in the sixth year of a presidential administration, the opposition party makes big gains. Even Reagan couldn’t buck that trend, IIRC. So this might be the year when a conservative candidate (who didn’t say anything stupid about rape, please!!!!) could win.
Doesn’t Maine have a lot of hunters? Maybe the RKBA issue will be important.