Bell just sacrificed the respect of at least some of his teammates and the chance to be an all-time great Steeler whether he's back next year or not.
It's his right to hold out, no doubt. He can make whatever business decision he wants, but it'd be a bad business decision for the Steelers to give him what he wants. He lost a chunk of his long term leverage when he was suspended twice earlier in his career, and he's gotten banged up plenty too. That's a result of his style and his usage, but that's what you get when you're an NFL RB like him. If he had stayed clean, he'd have been locked up already.
I hope the Steelers stick to their guns, and if Bell wants to play, great. He's not worth what he wants, and he'll find that out next offseason too.