I mean there's levels to not participating. Heat of the moment taking off I can understand more than just not showing up like Simmons. He's straight up pouting, but there is no way he will be cut.
Sitting out because you can't start seems pretty awful too. Andre did it, Wall and there have been others.
Lots of prima Donna behavior in the NBA. Guys refuse to enter games sometimes.
He got suspended which means he experienced financial consequences.
I thought the time he body checked the ref was worse than this, personally.
I think it's serious, and obviously he overreacted. I just don't see this as a straw that broke the camel's back moment.