I think that’s overconfidence. I agree that would be the best and most likely path should Biden drop out, but I am not a member of any organized political party. I am a Democrat. And several Democratic politicians have very, very large egos.
Trying to challenge Harris for the nomination in this scenario would be more suicidal than trying to primary Biden would have been, and nobody who matters attempted that. The names bandied about might not all be geniuses (hi, Newsom) but they can read the room better than that.