I agree with the "Mitch is trying to purge the Trumpists of power and swing control back to the traditional corporatist Pubs" argument. Trump was a useful idiot that could be beguiled, manipulated, and bribed to do the party's bidding (see Supreme Court Justice picks from the Federalist Society that were supposed to throw the election in his favor, but really are there to protect corporate rights). The folks who have taken up his mantle are true believers in a weird sort of populist cult of personality, and possibly harder to manipulate.
If Mitch can swing the news cycle during the early Biden Presidency to a Republican redemption story for the moderate republicans, then turn around and say "see, Biden/Dems wasted the first x weeks of his presidency chasing Donald Trump over YET ANOTHER impeachment after he left office over some silly little parade in Congress" to rope in Trumpists, he will have had his cake and eaten it too, and the Trumpist wing of the party will be back in useful idiot mode.