If Biden came out, Kamala would likely be in. No one really likes Kamala, she's just a resounding meh. And plenty of people are just plain racist.
If Trump came out, I'm not sure who would replace him, but nobody would have such a death grip on his base like he does. Many of them probably wouldn't even vote.
Not only that, more than most of Bidens campaign message is just warnings about Trump. If Trumps not the candidate Biden will have wasted billions and a lot of time on political messaging that doesn't matter anymore.
That's not really true. Mostly it's him struggling to describe all the ways that he's been one of the better progressive presidents despite having a completely non functional congress and outright hostile court. The only message that sticks is how awful Trump is because it's easy to say and understand, and is obviously true to some and obviously false to others.
Well that and he has been hostile to progressive platforms is whole time in office. So no wonder he’s not bringing attention to how progressive he has been.