I was under the impression that he was supposed to be handing off the role to Kamala, which was meant to be a shift towards a young(er) leader, woman and person of color that they hoped would check enough boxes for younger voters to make them think she was an actual progressive. That plan seems to have been nixed once they realized she has so little appeal across the board, she could probably lose a popularity contest to any number of infectious diseases, and Kamala 2024 got scrubbed.

They potentially could have found other viable alternatives, but they would have represented an actual shift in party leadership rather than bowing down to the party orthodoxy, so the DNC would never actually support them.

What they really want is another candidate like Obama that is charismatic and can talk in a way that convinces voters who don't feel represented by the current Democratic party to say, "This is the candidate who can deliver on reshaping the party in line with my views," while having no intentions of meaningfully deviating from the party line. I think the experience with Obama is still fresh enough in the minds of these voters that they are on guard against the same sort of deception a second time.