The writers do not hire and fire. They create characters. And neither Shawn nor Belle has had any sort of decent story since they appeared, in fact, no story at all. That was not Higley. Now yes, they may suggest someone for a character. Evidently it was decided to bring Philip back, and the search was on for an actor to take on the role, as Jay Kenneth Johnson was adamant about not returning.Yes, Griffith saw the actor somewhere, & suggested him to the suits. But THEY hired him, etc. NOT the writers.
O.K. Griffith leaves, Higley comes out of her "leave of absence", Ryan Quan is promoted, and bingo, the announcement is made of these actors being gone. Since Lani was already done, no surprise. The Summer story has been ill received, & I am thinking the suits could see that themselves. Shawn & Belle just were not doing anything, had no story, Belle already was getting a lot of negative comments about what she was up to since her return. It was NOT working. The decisions could have been made back in December or January, for all we know. And since Belle's last filming date is next week, it does seem that since she is not in scripts/story (written months ahead) that the decision was not just made last week.
We all can speculate, guess, all we want, but we really do not know what goes on behind the scenes.