Eric's characters on Sesame Street haven't exactly shrunk in on screen appearances lately. Steve's Ernie seems to pop up at least a couple times a season... Rudman's been there as well, but some of his characters haven't appeared all that much lately. I feel that has something to do with scheduling and not too much else.I'm pretty sure the performers are freelancers, so it's not a matter of Disney allowing them to do other projects. It would just be a matter of fitting everything into their schedules.
I'd assume that's the same schedule they'd keep in filming a Fraggle film, allowing down time and time for other projects.