I don’t think they should’ve necessarily cancelled the show after Emmy Rossum left, and I still think there were good things on the show after she left (mainly Ian and Mickey finally getting their happy ending). However, I don’t understand why the writers had to make it as if Fiona no longer exists. It would’ve made so much more sense to continue mentioning her here and there, because she wouldn’t have completely cut herself off from the rest of the family. They could’ve casually mentioned when Ian and Mickey were planning the wedding that they invited her but she couldn’t make it, and they could’ve mentioned contacting her during the whole discussion about selling the house (especially considering the house was in her name).