Who would it even be? Bernie yet may, and has a lot of influence, but isn't going to stand a chance against a Biden reelection. Buttigieg and Harris are already in the admin. Yang is discredited pretty thoroughly at this point. Warren is at such a high level in the party that she'd be committing long-term career suicide by rebelling like that, and would likely worsen the outcomes for her causes in doing so. AOC is actually too young to run even if she wanted to make that big a stink. Who'd it even be?
I'm not criticizing the lack of competition here. Biden's running, the dems are going to go behind him. They know how terrible the consequences of not showing unity could be. It's going to be Biden, and sane people better hope he wins.