I don’t think they’d be able to replace him prior to the election. That’d cause too much uproar, both within their party and without. What they would do is put in a viable candidate as VP, with the goal to replace him when he steps down after a few months in office. But they would only do this if they are confident the fix is in and they can manipulate the votes to get him elected.

Otherwise they’re just gonna flush Biden with a throwaway candidate like Stacy Abrams. I think they are waiting so long to decide because they are unsure if their usual tactics are going to work this time.