I don't want to lose Edna, they should recast. As sad as Marcia Wallace's death is, Edna can be played by someone else. Killing her off would be awful and cruel to Ned and her becoming a silent extra in group scenes would be a waste of a great character. I don't want Bart to have a new teacher or no teacher, I want to keep Edna.
I was fine with retiring Phil Hartman's characters, they were funny but not that important, Edna is different, she is one of my favorite supporting characters on the show.