I seem to remember an article about Jerry Nelson from a while back stating, among other things, his involvement with a folk band, as well as scaling down his duties with the muppets. These guys have been doing this for some time, we can't expect the older guys to stick around forever. Maybe it's for the best though, eric and steve's chemistry is much better than frank and steve. when you put the characters in new hands it lends to creating something new, rather than recapturing something old.