He wouldn't be running as an independent, he'd be running with the party he belongs to. The democrats don't really count Sanders as one of them, they've said as much openly during the primaries.
The 'lesser of two evils' option is what has American politics in the sorry state it's in, if half the people who are using their vote to keep the other candidate out were to instead use that vote for a third option that hasn't yet been bought out by corporate interests then there would be a revolution in American politics.
And what's to say those Trump supporters who are really just anti-Clinton supporters wouldn't follow suit?