So much to respond to in this thread...
The policy of the show regarding a regular actor's death would depend on which one died. Julie Kavner already has Tress MacNeille as an official back-up, and she's quite good from what I've heard (anyone remember the Aspen performance?). Nancy Cartwright and Pamela Hayden have similar voices and could replace eachother (not to mention the fact that Maggie Roswell could probably step in for either and that she's on good terms again). Remember how many times each _has_ stood in for eachother in the past (and let's not forget Russi Taylor). Dan, Hank and Harry could probably take each other's place if the situation were to demand it.
Let's also not forget how completely recasting an actor's roles _has_ been successful. South Park lost ALL of its female voices upon the death of Mary Kay Bergman, but the show found replacements without losing a beat. Marcia Mitzman Gaven, given enough time to rehearse, _could have_ been an adequate replacement for Roswell ("Little Girl in the Big Ten" had a nearly flawless Miss Hoover appearance). If one of the more major actors died, there would be quite a shuffle, but I doubt that would show on camera (or mic, as it were). The talent search could very likely yield adequate replacements. Remember the fact that Moe and Mr. Burns are replacements as it is!
And if anything, Robin Williams could take over as Homer, just to get Dan back over that whole Aladdin thing. ^_^
-- An XPeeple Production -- "As a child, I always wanted to bite his head off. This is my way of doing that... creatively." -Andy Dick as Marylin Manson "Most critics want to be authors, screenwriters and novelists, [but] they just suck." -Joe Rogan