I think Sesame Street is for children from ages up to six or so. Spending as much time on Elmo as they do, in my opinion, drastically shrinks their audience. Children who would watch Sesame Street at age 5 find Elmo dull and switch to something else.

For sure, my evidence is anecdotal: my daughter enjoyed the show from ages two to three, maybe a year and a half total, and now has zero interest. She still finds Elmo cute and cuddly but gets bored by his segments way before they end.