The Dems will nominate Biden, then put someone on the underticket (Hillary? Booker? Harris?) who they think has a better chance of winning. If Hillary, the Clintons will find some way to assassinate Biden. If Booker or Harris, the party will suddenly discover Biden is mentally incompetent, remove him from the ticket, and put Booker or Harris in Biden’s place.
Biden could make a deal with Hillary: if you don’t kill me, I will promise to resign after one year and you’ll be President. (But Hillary may insist that it be after six months.)