Agreed! It makes no sense to me there are so many ways they could have kept him around, he could have been like Feeny and keep following the kids to different schools, or become a college professor to explain limited apperances and leave the door open to him becoming a regular again once the kids go to college, he could have appeared as a friend of the family, he could have continued appearing as Shawn's parental figure- and actually the actor had been told just two months prior to being dismissed that in s4 there was going to be more exploration of his relationship with Shawn but then he got the sudden call about his diminished role in s4 and being written off.