It sounds like you just don't like the show, period. There's plenty of ways they could write a more satisfying ending for a major character without it being a "fairy tale ending". When Ian was written off they sent him to prison, after a full day of saying goodbye to everyone, only to discover his celly was his longtime boyfriend. When the Mom was written off, she died, and they spent a good 10 minutes of screentime having the family that she abandoned eulogize her and reminisce. Both characters had story arcs leading up to the end that lasted multiple episodes.
Meanwhile, Fiona just slinks out of the house while everyone is away, hops on a plane to who knows where, and has her last conversation with some random lady on the plane. Her character's farewell could have been done better but there was no way they would have killed her off.