This is more of a fanfiction type of answer where one tries to explain it in story context. However, perhaps the menu and theme changed.
I can't always see real well who's int he background, but you say there are still kids in there, with muppets also. So, perhaps the number of older adults in the neighborhood declined (just figuring that it was supposed to be more than just cast members - therehave to be toher families w/all those kids). And, at the same time, the number of younger people with more kids increased.
So, the menu and such just had to change with the times. Instead of serving big meals with larger portions, Hooper's went to smaller portions and more kid-friendly stuff. Perhaps it's more like a Chuck E. Cheese/C.C.'s Pizza now, though with other stuff, too. So, it's more attractive to kids. One could even argue that the kids and muppets in and around Hoopers make it so crowded the adults have to wait too long, too.
I don't know if you wanted that or a more "why did the writers do this," but as for a story idea it is quite plausible, as some restaurants to change menus, decor, etc. to match the people coming.