The guys who played Mr. Macintosh and Willy the hot dog man probably wanted to move on to other things, like Frank Oz, and Jerry Nelson. The last time I remember seeing Mr. Macintosh was the early 90's, and the last time I saw Willy was 1985 (I barely remember the guy) I think when people get older they just want to relax, and spend time with their families and travel. That's probably the case with Willy and Mr. Macintosh. It would be nice to see Mr. Macintosh make a comeback though. With the new Healthy Food theme the show has, he would work perfect in an episode with his fruit and Vegetable stand. Bob may be 72 now, but I think he really takes pride in his work and is very dedicated to the show, and is probably somewhat attatched to it too. Heck 36 years is a long time and after that long of time, it would be hard to just get up and leave. I found out the actress who plays Susan is 67 years old now. We get to see her usually once a year or so now. (kids probably barely or don't even know who she is) But it's probably because she's getting older and is taking time out for other things in her life too. Emilio Delgado turned 60 recently. After 34 yearsh e's still going strong as Luis and I think he really enjoys the show as well. I guess it all depends on the person. Some feel it's time to leave the show and move on once they hit a certain age, and others are determined to stick with it, until they can't stick with it anymore. Hope this helps.