yeh it's not zero. It just relies now on something insane, like Jesus endorsing Bernie Sanders tonight, or Clinton getting embroiled in a scandal with Huma Abedin, or aliens abducting Hillary. You're totes right, not zero percent. But I think it's safe to say when your best odds of winning rely on the other candidate materializing Medusa snakes in her hair, it's over. By all means, stay in as long as he likes. Bernie supporters, burn them votes in the furnace for the "message." Hillary is gonna pivot to the center the second the primary season is over anyway, so for naught. She's not choosing him for VP, being strategically worthless to her.
So we're having this conversation wherein nothing meaningful happens, but at this point it is sheer delusion to believe Bernie has a real shot.