I honestly don't think the performers have much to do with it, they generally don't have that kind of say. The people in charge of the show saw that the character was popular, they listened to certain child experts (not all of which I agree with), and they ran with it. And now they're stuck with it.
The only way a perfomer has any control over how popular a character is would be the performance of making that character endearing. The problem stems from what you just said, not to mention the popularity of the TME dolls back in the 90's, which really spurred the Elmo-centric style of the show now.
That said, You all know the real reason why Elmo speaks in 3rd person, right? The original version of the character was a lot more monstery than he is now. There are three kinds of Monster speak in SS/Muppet world. Correct English- Herry, Grover, Rosita (with correct Spanish as well), incorrect stitled English- Cookie Monster, Animal (he's not a monster, but he counts), and varying degrees of unintelligible mumbling- Two Head, Frazzle..etc. If you see Early Elmo skits, he had a Baby Monster stilted English speach. So that's pretty much the origen of his third person. With that one exception of speaking like that, his speach evolved to be more child like, and not so much monstery.
That said, clearly Elmo is a marketting tool now. I wouldn't be shocked to hear more parents ask their 2 year old kid if they want to "watch Elmo" and not "watch Sesame Street." Stuck is indeed the word, herald. SW deep down wants to move away from it, hense why 2 episodes last season didn't feature Elmo's World. They wanted to see how kids would react without that segment. And they reacted badly. They wanted Elmo. It takes 2 groups to make something a phenomonon- the creators and the mass audience. The mass audience wanted Elmo, so they got it. Now SW is indeed stuck having to use Elmo as a marketting tool. And it really is a double edged sword.