Fan Casting Demoted to Extra in Sort by Tropes

We have a member of the supporting cast. They are important to the plot. Sure, they fall Out of Focus at times, but they're still pretty relevant to the proceedings for the most part. But then comes the sequel, an adaptation or The Movie, and... hey, where'd they go? They weren't completely written out, but didn't keep what importance they had in the original. The character's fanbase will complain, while everyone else may just accept it as the new status quo.