I do agree that there needs to be main performers and official under studies. Sesame Workshop handled their recasts far better than the Muppets did. Elmo, Count, Oscar, Big Bird, Bert, Grover, Cookie Monster, etc. None of these characters missed a beat when their original performers retired. I think that Dave needs to start training an understudy for Gonzo if he isn't already. And maybe we should have certain characters switch between the understudy and the main performer like they did with Big Bird just to get the audience use to it and gauge reactions.
Ideally, all the performers would live forever in perfect health. Realistically, there have to be contingency plans.