In '16 Sanders supported Clinton, he had no other choice. Today he has to support Biden, albeit grudgingly.
Sanders has no choice but to work with the democrats, he caucuses with the democrats. If he refused, he’d end up being kucinished: sidelined, driven out of office and of no help to anyone.
Further, Sanders running as any sort of 3rd party candidate would ensure he could not win. He’d take votes from the democratic candidate too, ensuring the republican won. He’d be even less popular with the democratic establishment than he already is.
Sanders did not win the nomination in '16 or '20 because the ruling elites in the democratic party would not and will never allow it. That’s why they shoved doddering, senile, foot-in-the-mouth, sniffing females hair, cardboard, over-scripted Biden out into the spotlight.