Y&R, more than any other, has longtime cast members who have been with the show 40+ years, which adds to their "comfort food" feeling. Someone can tune out for a few years, then return to see so many familiar faces still in the mix. This makes it much easier to draw a lapsed viewer back in. Oh, Nikki's still fighting with Ashley. Oh, Victor is still an overbearing jerk. Oh, Sharon is still a tramp. I think that if a chunk of those veteran actors were to leave, their reign as #1 soap might be jeopardized because soap fans tend to hold on to love of characters, not necessarily love of stories, and these characters are so identified with their actors that recasting just would not be an option.