I think Sanders could hypothetically win a general election but I wouldn't argue that he's super electable either. Hillary does have him beat on that front. However, I think the benefits of Sanders winning the nomination and and moving the political debate to the left (even if he loses) FAR outweigh the modest benefits of a second President Clinton.
But as we know, it won'e be as simple as President Sanders=socialist dream. We need someone who can work with congress, someone who has experience in the white house. It's like thinking President Cruz could somehow cut back everything.
I mean heck, Hillary isn't that bad. 10 years ago she'd be considered on the liberal wing, and we'd be fighting off the nutjobs like Lieberman and Bayh.