Show quality aside... these sitcoms are very disposable and last only a few years no matter how popular it was to start with. Personal feelings about these shows aside, I hate that kid's programming is so disposable. Even when it's popular and does well and it has a sizable audience, it always has a production cap. Disney LOVES production caps, apparently. That's why Darkwing Duck ended with a time travel trip to wacky dinosaur land, and nothing more fitting. They didn't want to produce another season, simple as that.
The Muppet appearance has nothing to do with anything. It was nothing more than cross promotion, and if it means just one or two more ticket sales come next March, it did its job. Something tells me they were going to end the series soon anyway. Like I said, because it's disposable, and there will be another show that's similar coming off the conveyor belt.
But I agree. I'm very happy with the line of cartoons, both Disney and Disney owned Marvel made. They've found a far better balance between the two than they've had for a long time. Heck, they're not even trying to make the cartoons take a dive for live action programming dominance. I liked Emperor's new School and The Replacements fine enough, but something about them seemed like Disney didn't care if they succeeded.