He wouldn't win a single Republican primary state and would be utterly irrelevant nationally.

Charlie Baker didn't run for governor in Massachusetts, where he's enjoyed a 74% approval rating, because he was going to get primaried by Diehl and would have to spend a fortune winning a GOP primary that his own state party endorses his opponent for. He'd probably end up losing that primary even though Baker has consistently been the most popular state-wide elected official in America.

If Baker doesn't get a post as an ambassador or something, Baker is going to cash in as an advisor and board member. He's done with electoral politics. He's unelectable in Republican politics.