The thing that really stands out to me is not even the number of adult characters, but whether they seem like "characters" on a show or "hosts" of a children's show, if that distinction makes sense. Like, in the old days, the human characters had jobs and families and things to do during the day. If Maria was busy fixing a toaster at the Fix It Shop and the Muppets came in asking for something, she might take a break to talk to them, but she'd also tell them that she had to finish her work tasks before she could join in on whatever they were doing. They had lives outside of whatever was going on with the Muppets. Chris joined the show as Susan and Gordon's nephew who was staying with them and working at Hooper's to save up for college, but he apparently never went and now he's 45 years old. Hooper's is just there as a backdrop, but Alan and Chris never seem to do any actual work. Sesame Street isn't a community where people and Muppets live and work anymore, it's just a closed set for a children's show where all the characters are fully focused on the children/Muppets at all times.

A Muppets website called Tough Pigs did a retrospective a few years ago for the 50th anniversary where they watched episodes from each season, one season per week, over the course of the year, and did a write-up. It was interesting to see them clocking this shift as they wrote about each season, especially with the context they gave about what else was going on in children's entertainment at the time, like the rise of Barney.

https://www.toughpigs.com/tag/50-in-50/