Evolution is something that cannot be helped. Let's forget the post-Jim products and look at the massive evolutionary changes since the Muppets were created. We wouldn't have a TV show. We'd just have little novelties where they mimed to current songs, versus the huge thing thing the Muppets became. No Sesame Street, No Muppet Show, and definitely no Fraggle Rock (and as much as I love the Muppet Show characters, Fraggle Rock was a very special thing that owes itself to 80's Jim).I wasn't aware they needed to evolve after The Muppet Movie. No need to evolve after hitting perfection.
TMM was a completely different movie from the later MTM. And both films are great because they're both different. TMM has a lot of wacky "no fourth wall" humor and some very Jay Ward inspired puns and running gags. MTM was more of a special, indie film feel that didn't need to make constant visual puns. That's evolution.