I've been saying for a while now, the show feels a little...deserted. Sure, they always had episodes where there were just a couple humans and Muppets, maybe one of each, but those were getting more frequent. And a lot of the time in recent years human characters are supplemented by celebrity guest stars. Sure, we'd get some episodes where the street was busy every so often with as many cast members, puppeteers, and extras as they can shove onto the set. But for the most part, they've managed to make bottle episodes of a show that's essentially always a bottle episode show. made them more bottley.
Heck, I don't think I've seen many human kids on the street this year either.
This sounds like it comes down to many things, ageism may be a factor, but the way I see it is this...
It's clear that Sesame Workshop is trying to scrimp all the money they can, and it seems a huge factor in Chris and Alan being on the show the most also has to do with availability, and trying to get as many episodes shot in as few days as they rent the studio as possible. When the older cast members just come in one day to do their routine "we still live here" 30 second cameo in one episode, it's not really cost effective. If Bob had a lot more screentime in recent years, I'd be far more annoyed by it like I am Luis (who really wasn't on the show until a season ago anyway) and Roscoe. I'm not saying it seems like those actors were moving on, but Loretta had a life outside the show and that's why she wasn't around much. Oddly, it seems that Luis and Susan were making small comebacks, making this come off a little nasty a move. As for Bob, the audience won't realize he's gone if he's just that guy who pops up every so often in the background.
Still...let them pop up every so often in the background. Sure, it's cheaper to let them not appear on the show, and a chunk of the audience in the age appropriate demographic thinks every Muppet is Elmo...they freaking think Mr. Potatohead from Toy Story is Elmo! I'm not kidding, I come from a bizarre experience. And to me, that's the bigger bias. The bias for characters that are easily marketable as plush toys and DVD's without having to pay for actor license rights. Remember, they call it show business for a reason.