I agree, you have to start young to get kids to like something. But it's just not true that no kids today enjoy the Muppets. It's that they're not even being given the opportunity to judge for themselves. At the TV Museum, a little girl was genuniely enjoying Muppets Go the Movies. But when will she get another opportunity to watch it in regular life? The answer is not to simply bring in new characters (no offence Pepe hehe), but to make the original material more available in the first place.
As I just said in another thread, there is this movement to repress any of the old material and claim it's just not useful anymore, despite evidence to the contrary. It's been an entire decade and the new material (while it has some good points) did not keep the franchise alive (and that was before Disney). Sometimes when times change, the answer is not to roll with them.
I speculate that if more attention had been paid the original projects, maybe the Muppets would not be a power house today , but they would be familiar to children and fondly remembered. As it is now, the new projects have not gained huge acceptance and the audience isn't able to view the old shows. Thus, the Muppets are practically out of the picture.
I don't think the Muppets will never be appreciated again. Disney was finished for two decades, but then got a huge resurgence in the early '90s. These things go in cycles. But Disney didn't try to completely change what made them popular in the first place.
Everytime Kermit tried remaking himself in MTM, he failed miserably. I can't imagine Kermit the character saying "Sometimes you just have to give up the things you love in order to change with the times." You know I mean, the world is tough sometimes, but you can't give up and give in.