Apparently she already wrapped all her filming for Episode 8. My friend Espiga had the idea that Episode 9 could start with a giant space battle, Leia's ship gets destroyed by the First Order, and then we have a funeral, with everyone looking sad. Don't even show her, and no flashbacks. (Speaking of which I just realized Star Wars rarely if ever does flashbacks, Rogue One showed child Jyn but then did a timeskip to her as an adult, so...) The most respectful thing to do. I don't think it would be right to recast her or cgi her. Of course, I'm sure they hadn't planned on killing off Leia, since Han already died, narratively speaking, but it can't be helped. The real Princess has passed away. :'(