Biden is the rightful nominee, it's who the people, at least the Democrat ones, wanted. The time to choose someone different was in the primary.
Yes, so rightfully, Joe steps down, Kamel takes the presidency and she also can name her VP -- this is where I see Big Mike entering the picture OR -- she may be paid a visit and told she needs to step down as well.
I don't know what the DNC rules are regarding their ability or rule changes to insert someone else.
I'm not sure how this is going to play out, but it is looking like those that voted in the primary are not going to get their choice.
Then there's RFK .... though I don't believe he has been able to qualify to be on the ballot in all 50 states.