Yeah but then he got Kamala Harris as VP because Obama likes her, but she has negative charisma so the whole "let's cultivate a new candidate" plan never went into motion.
It's very generationally fitting that there is no alternative candidate for the very fact that the older dems have made absolutely sure to hold onto their positions regardless of outcomes, popularity, or senility, and have thus stunted the development of anyone else that would replace them.
Two of those candidates are complete outsiders, and extremely unserious.
(The actually serious one, who could definitely be president at some point, is also the one who took a position in the administration. Funny thing.)