The way things stand now, a split would mean that McConnel et al leave to create their own party, not the other way round. I doubt they'll do it despite all their rhetoric. Like never-trumpers in 2016, they'll quietly sneak back to kiss Trump's ring.

Even with a split, given polarization and gerrymandering it might not be all victories for Democrats. They might even end up losing some swing seats they recently got if there are two parties to compete against and some independents flip back to supporting whatever moderate Republican offshoot might emerge.