I love how this has turned into the Walter thread.
It is a valid question though.
Even though the writers confirmed he has a part in the movie. But who wants a subtext where he disappears? Like that little girl who disappeared off of Family Matters when her mother wanted more money. They never wrote her off, and that leads you to think something dreadful happened.
If Walter were to disappear completely, that would mean 2 things... either Kermit broke a promise (something pretty lousy to do to someone that saved your little entertainment troupe) or that he didn't find his dream that fulfilling and he just up and left. Somehow, if they just couldn't use the character anymore for whatever reason, I'd love to see him written off as being too shy to perform still, and working quietly as an important behind the scenes role.