IMHO they kept Schulz doing Peanuts long after he should have retired. I remember in the 90's looking at the current comic strips and thinking, "Why is this so popular? It's not really that funny." But then I found some books of Peanuts cartoons my Mom had from the 70's (or maybe 60's), and they were positively hilarious.
Bill Watterson made the right decision by retiring from Calvin and Hobbes at the peak of his game.