He's definitely lost some money, but not much. I think the report that he stopped being fined once he and the team got on the same page about his mental health treatment is the correct one, or things wouldn't have gotten so quiet.
He can also afford to lose his entire salary. When this all started someone suggested Simmons' recent purchase of a $17.5 million mansion would put pressure on him but even if he paid cash for that he's still not without $, and if he has a mortgage that that is no more of a burden for someone who makes almost $30 million than the monthly payments on a $50,000 double-wide would be for someone who makes $100,000. He might not be able to go multiple years w/ no income, but one, easily.
And the Sixers can also afford to waste the money - and more than they can afford to take a deal they don't like.
Not sure what the league could do, or that it would accomplish anything. The team could file a grievance claiming his mental health issues are dubious. If he didn't have the mental health card to play he'd probably have an injury (and they could also file a grievance). But nothing would get settled this year and I don't see how you could change the rules to legislate either thing in advance. He got fined when he was refusing to cooperate with the team and show up for training/rehab/workouts. Now he's doing that.
From the Sixers perspective they probably also feel that fining him, and the accompanying noise/conflict around it, would only hurt their trade options more.
Another nuclear option would be to void his contract but then you get even less for him than you did for the other failed Process guys.