What makes you think any of them want to run for president when they didn’t even take part in the primaries?
Also, why do you assume Sanders would do it when asked? He’s 82. He’s a lot sharper than Biden, but he’s still 82. His energy is going to be limited. I would be very surprised if he even sought re-election to the Senate at this point. He probably wants to spend his last years with his grandkids in Vermont (or wherever they live), not arguing with people in D.C. And I’d say he deserves that happy end to his life rather than spend what may be his last four years dealing with a bunch of hostile bullshit.