The humans gave the show realism, but the Muppets give it longevity.
The Electric Company lasted, what, like 10 years maybe including reruns? Square One Television lasted until 92 after premiering in 87. I'm not saying these shows are relatively unheard of, but say "Cookie Monster" and everyone you meet will say "OH! Yeah, I remember him!" I fell off the chair I was sitting in when I watched a Fresh Off the Boat where they referenced "Mathnet." And I almost completely forgot about 321 Contact.
The Muppets indeed made the show more profitable, more memorable, and more approachable than it would have if Sesame Street were just humans and cartoon skits. Let's not forget the marketing. Human characters appeared in books well through the 80's, but for the sake of marketability and easy cultural translation, this shifted to be more Muppet based, and human roles were now taken by adult monsters and AM's. And let's not forget the international scale. While some humans appeared in some of the dubbed skits, it's just less awkward to pull off the illusion that Ernie and Bert live there in Germany with their human characters who they never actually interact with.
I do agree the humans and Muppets balanced each other out nicely, it's just we saw a
lot more human skits before Sesame Street really took off with a larger roster of characters. Like those increasingly terrible sitcom comedy duos that thankfully disappeared within a few years of the show. Replaced by more monsters, more parody characters, more Am kid sketches. We saw a decrease in human alone skits since the late 70's into the 80's, but they always remained a staple in the street stories with impeccable balance.
Yet, we come to the time paradox. How many times did Bart and Lisa Simpson go on summer break, only to come back to the same exact same grade in the fall? How many Christmases did they have? Bart's been 10 so long that he'd be in his 40's in real time. But its a cartoon. That can be pulled off easily. Not so much when Elmo was there at the birth of Gabi, and he's still an audience surrogate 3 and a half year old when the actress is old enough to have kids of her own Elmo's age. But there
is one thing we're forgetting...
The Fleeting Demographic Rule
The show has a (and this is a very generous estimation) 2-6 year old turn around. Kids watch the show for about 4 years, grow out of it, probably watch it a good 20+ years later if they have kids of their own. Would they care who Bob is if they've never seen him in an episode before? Let me put it this way...Trix. 90's kids complain hard about how the cereal is no longer fruit shaped
when the cereal was never fruit shaped to begin with and actually reverted close to the original version. Generations are all possessive about things in their generation. All I can say. The kids with kids watching now...and wait for it...it gets pretty hairy...
grew up on Elmo's World. Elmo's World alone is 20 years old!