Former President Obama faced a major challenge last night on the third night of the Democratic Convention, in trying to persuade the American people that he sincerely believed his former Vice President Biden was truly up to the task of being president. Recall Obama passed over his VP Biden of eight years in 2016 to support his preferred candidate Hillary Clinton. So obviously Obama did not think that highly of Biden's political skills at that time in preserving his (Obama's) political legacy.
Obama also was lukewarm to Biden running for president in 2020. Obama withheld his endorsement until Biden was literally the last man standing. Or stumbling as the case may be. Clearly, Obama would have preferred someone younger like Senator Booker or Senator Harris or more vibrant and policy-oriented as Senator Warren to preserve his historical legacy.