I don't think the explanation needs to be that convoluted. The parties in a two-party system end up very hetrogenous coalitions, and there's a lot of local variation. Manchin has been more of a (West Virginia) Democrat than a (West Virginia) Republican. West Virginia is about as red a state now as Vermont is blue.
The main qualifications for being an elected official affiliated with a particular political party are "able to win the primary" and "subsequently able to win the general election". Kicking someone out of your coalition because they only support you some of the time is extremely counter-productive unless you have a better alternative, and even then it generally makes sense to wait to actually replace them instead of telling them to get lost in advance. But you need to have a replacement with those two key qualifications, and looking at the election performance of Manchin's 2018 primary opponent, it's not at all clear there is one.
(If Manchin were looking to hurt Democrats and advantage Republicans, "let Schumer set the Senate agenda, then undermine Biden by voting for a bunch of stuff, but not everything, that Schumer and Biden support and McConnell opposes" seems a pretty poor plan compared to "make a deal to switch parties immediately after the 2020 general election, preventing Democrats from driving turnout for the Georgia runoffs with the prospect of a Senate majority".)
no subject
The main qualifications for being an elected official affiliated with a particular political party are "able to win the primary" and "subsequently able to win the general election". Kicking someone out of your coalition because they only support you some of the time is extremely counter-productive unless you have a better alternative, and even then it generally makes sense to wait to actually replace them instead of telling them to get lost in advance. But you need to have a replacement with those two key qualifications, and looking at the election performance of Manchin's 2018 primary opponent, it's not at all clear there is one.
(If Manchin were looking to hurt Democrats and advantage Republicans, "let Schumer set the Senate agenda, then undermine Biden by voting for a bunch of stuff, but not everything, that Schumer and Biden support and McConnell opposes" seems a pretty poor plan compared to "make a deal to switch parties immediately after the 2020 general election, preventing Democrats from driving turnout for the Georgia runoffs with the prospect of a Senate majority".)