I think it’s likely Harris would be the nominee if Biden stepped aside, unless they are spooked by her polls, as well. But that is a lot of ifs, and really doesn’t bear much weight on the likelihood as I see it (and hate it) of Biden staying in the race and losing to Trump.

But yeah, that’s why it mostly hinges on Biden, because the DNC has rules in place to protect the presumptive nominee from a delegate mutiny.