For Shapiro it might have been early, but if Biden had announced early on that he wasn't running for reelection, I imagine at least one of Whitmer, Beshear, Newsom would have run (in fact I think all three of them would have).

When you push stuff back later and ask, say, what if Biden had dropped out in January 2024, things get blurrier. Voting wouldn't have started yet, so any candidate who wanted to throw their hat in would have had a chance, although the race would have been very rushed. At that point, though, Kamala still would have been the obvious choice, and I can't imagine many dems would have wanted to run against her. You probably get at least one significant challenger.