I speculated at the time that he was being put up as the 2020 version of pathetic losers like Walter Mondale and Bob Dole.
The timing of the whole 2020 primary process feeds my suspicions. The Democrats cleared the decks for Biden on Super Tuesday — which was March 3rd. Within two weeks the whole political landscape blew up with the COVID hysteria, which put the previously inevitable Trump re-election scenario suddenly in doubt. By then it was too late to change horses and go with another candidate who had more charisma and a brighter future instead of the broken-down old loser they had pushed onto the stage.
Your analysis makes perfect sense. Biden was at the end of his career, and was the perfect throwaway. People tend to ignore the dynamic and opportunistic character of politics.