I think your maybe underestimating Biden here.
I would suggest almost every other Democrat who was running could easily have been Hillary Clinton 2.0 in that election. Imo
Kamala Harris loses Wisconsin, Pennsylvania.
Pete B loses lots of Latino and African Americans and other socially conservative democrats. Definitely loses Georgia and Arizona, Pennsylvania.
Bernie loses many economically conservative voters.
Amy K maybe competitive but not a very likeable presidential candidate. Probably doesn't turn out the African American vote as well as Biden. Loses Georgia, maybe Arizona.
Cory Booker was maybe the best general election candidate they had, but he just didn't stand out enough.