You don't think a solid conservative senator would be better than a Democrat turncoat?
Plus he'd have President Trumps backing.
I know Trump has picked some losers in the past but I don't see him backing a non-conservative.
I suspect if Manchin bolts on the crazy Dems, he will choose to caucus with the Democrats anyway thereby leaving the Senate in the hands of Chuck Schumer. If he decides not to be the pantywaste he seems to be, and join the Republicans then he would get his choice of chairmanships and in a state that went 70% for Trump should have no difficulty in 2024. If he stays in the Schumer - Pelosi - AOC insane asylum he will likely be gone in 2024.