He can’t win the general election (I believe that). Their overriding goal is to win it. Therefore they will stop him at all costs if he turns out to be a real threat to Hillary. The vast majority of the far left will fall in line and vote D like good little soldiers next November, when Sanders’ campaign will be a distant memory. Sanders himself, unlike Trump, has ruled out a third party bid because he doesn’t want to see a Republican elected.
Knowing her history, "an offer he can't refuse" wouldn't be out of the question.