For some reason this reminds me of Game of Thrones. Specifically how in the early seasons - when they had books to base it off of - every character had a good story reason for existing. But once they ran out of GRRM material to pillage, they basically fell into the classic pattern of TV writers. Things like having characters hang around all season and do nothing just because they were main cast members, or having characters formerly halfway across the world from one another run into one another to form some sort of "avengers" style supergroup.
Ultimately, this is probably one of the ways that fandom kinda ruins storytelling. Either the showrunners discover they really like a character - or the fans really love them - and then all the sudden rather than flowing organically the plot is thrust forward partially on casting concerns.
I agree entirely. It's much like a professional sports franchise. You need to be focused on the betterment of the overall story (or, in the case of my metaphor, the "team") rather than be emotionally connected to actors / characters ("players") who don't fit in any longer.
Sometimes you have to make the tough call.
Most serialized shows don't know how do make that call.