Sanders basically has to either withdraw his presence from 2016 presidential election and re-purpose his movement into local elections or go independent to maintain the movement.
That's just it, though, dude -
In November 2015, Sanders announced that he would be a Democrat from then on, and will run in any future elections as a Democrat. On February 4, 2016, Sanders said, "Of course I am a Democrat and running for the Democratic nomination."
Sanders' party affiliation is younger than my driver's license. "Going independent" is hardly a tribulation for him. If he decided to hook up with the Green Party they'd be viable for the first time since 2000. If he decides to retain his senate seat he gets to be Pelosi's counterbalance. The only way Sanders goes away is if he decides he wants to go away and makes the active decision to fuck off with all of it.
And if you don't think Barack Obama doesn't email everybody that donated to his campaign every day you clearly didn't donate.