It would be a waste of time to attack Biden when you don’t know for sure he will be the opponent.
If Biden survives (unlikely imho) there will be plenty of time to blast him.
If Joe really wants to stay “in”, and he seems he does, with a lot of bullheadedness involved, it’s going to be very difficult to get him out. It’d take the defection of a really prominent Dem, or maybe the majority of House and Senate Dems, to put on enough pressure. Single Dems would be either Obummer or Heels Up. MAYBE Schumer (and I’m sure if he turned there would be numerous Dem Senators lined up with him.) MAYBE White House Chief of Staff Jeff Zients along with some other high level staff? But I’d be highly surprised if Zients flipped - he’s at the core of the present cover op!