Absolutely nothing to lose, fundraising money to gain (just don't take on any debt):
Michael Bennet - not up for re-election until 2022
Steve Bullock - termed out, hard to go from executive office to being the junior seat in the Senate (as much as I want him to)
John Delaney - doesn't currently have a job
Joe Sestak - out of a job, best chance for people to not forget who he is
Tom Steyer - has nothing to do but burn money. And he has a lot of it.
Marianne Williamson - nothing to lose, name recognition will help sell her books.
Kirsten Gillibrand - not up for re-election until 2024. Hmm, run now, then again in 2024?
Bill de Blasio - termed out in 2021. Nothing else to do?
These two however, it's a fair question:
Tim Ryan - will drop out as soon as his House seat is in danger. Right now it's safe.
Tulsi Gabbard - will drop out as soon as her House seat is in danger. Right now it's safe. Also wants to be Bernie's VP candidate.