I don't think you can say what Bernie would do if Biden stepped down, he has stated that he will not run precisely because Biden is running again and he doesn't want to make any voters think he was an option, the incumbent is just automatically going to win the primary. He doesn't want to sour any voters that really wanted him to win the primary when there is no chance he will win going up against an incumbent. If Biden stepped down and it was an open DNC primary then I think Bernie would almost certainly run again.

You are being weirdly stubborn in considering actual possibilities. I get that we are likely stuck with Biden but that's because Biden is going to run again, not because there are no other options. Biden is not a good choice though and he should honestly step down, there are way better choices. Newsom would also be better than Biden and you could be sure if there was a primary with no incumbent in it he would be in there.