Williams makes roughly as much as Mo does over the next few years...and he isn't nearly as good right now. Does he have the potential to be? Sure...but it seems like he would have started to show something by this point, and averaging less than 10 a game while starting every single game isn't showing anything.
The NBA has become more so about potential than actual talent, especially the draft. Your always going to see some busts in any professional draft, but come on...the NBA draft seemingly is about 75% busts every year now, with only the top 5 players making any significant impact first and second year.