There are plenty of well-known, viable Democrats who could have run if Biden had chosen not to run for reelection, like Gavin Newsom, Gretchen Whitmer, or Pete Buttigieg. The problem is that I highly suspect these ambitious, well-known Democrats won't want to be simply handed the nomination at the convention instead of campaigning for it and properly winning it like every other official party nominee has done. Getting into office simply because they were the last-minute replacement for Biden on the ticket would hurt their mandate and overall legacy. I could almost see a largely unknown figure, someone who ordinarily would have had no plans of running for president, taking the nomination and running against Trump on the grounds that they're, well, not Trump. I doubt the convention would want to take a risk on an unknown, though.