The third option player on any championship caliber team is going to garner a decent salary. You gotta pay what other teams are willing to take away from you. He is slightly overpaid but his salary hasn’t hindered them from getting talent and his play isn’t the reason they haven’t won.
He averages 17/8/3 shooting 86/47/35 in the playoffs. Plenty of teams are going to pay $30 million for that.
Again you put any other mvp caliber big man (Jokic, Giannis, and AD) on that Sixers squad these last few years and they at least make the finals in that east if not outright win a ring.