This argument implies that Biden is left of Obama. It also ignores the very real risk that by not voting for Biden in this round there will be no fair future rounds to participate in. It also ignores all other elections and chances to influence the party.
Trump is a uniquely different candidate than McCain, Romney, Bush, whoever you want to pick. He tried to interrupt the peaceful transfer of power and there is little reason to think he would not try again if given the chance.
Real life is not so simple as game theory and attempting these simplifications is not as rational as you imply.
Please vote in the coming election for Biden. The consequences of "Punishing" the Dnc are too dire to ignore.