In their younger days, the Stones were motivated by a variety of interests: money; fame; anger; drugs; women; and the desire to prove that they were the best band in the world. As old men, all of that is gone. The Stones continue to perform because they still have the music in them, and because it makes them and their fans happy. Any other kind of ambition is irrelevant at this point in their lives.
So it is with Biden. You have to believe that he was totally sincere when he said that it should have been his son taking the oath. He has no need for power or an ego boost at the age of 78. He only ran for president because he thought he was uniquely positioned to save the country from Trump. He was right, and we have cause to be grateful.