For a reference essay on the shift from Big Bird to Elmo:
https://kotaku.com/how-elmo-ruined-sesame-street-1746504585
Major thesis:
"It started in small ways. A lot of stories used to be based around Big Bird, who was psychologically characterized as a six-year-old. And in a lot of ways, this was ideal—six years old was on the older end of Sesame Street’s age demographic, and Big Bird served as both a role model and a peer to his audience. Big Bird’s comparative maturity also meant that he could project a wide range of emotions—he was ‘old enough’ to understand mature concepts, such as the permanence of Mr. Hooper’s death."
"Elmo stories, on the other hand, tend to affirm and celebrate the child’s perspective. Rarely, if ever, is Elmo’s innocence challenged, or is he forced to think about someone’s happiness other than his own. He spends most of his time hanging out with Zoe, Abby, Telly, and Baby Bear—Muppets who share his emotional maturity, and unlike Grover, Kermit, and Ernie, do nothing to push him. In fact, he is the de facto leader of his group—the dialogue lowers to Elmo’s level, rather than rising to an older character’s. And while this is cute and fun, it gets old fast, and it doesn’t really go anywhere. Elmo is learning about counting to four and different shapes, but he’s not learning a whole lot of life lessons."