Which is why, in so many cases (this not being one of them, mind you), it's better to simply replace a character altogether, as opposed to trying to recast the character with a different actor or performer: that one actor is the one who really breathed life and soul into the character, is if something happens to that actor, and you try to recast the character, you're setting yourself up for disaster. That's why shows like M*A*S*H and Cheers were successful despite their cast changes, because when actors left, they replaced the characters altogether with new characters that, while they differed significantly from their predecesors, still were engaging to the audience. Somehow, nobody minded how Gordon was recast three times, but can you imagine if they tried to find somebody else to play Mr. Hooper after Will Lee's passing?
Again, it's a whole different ballgame when trying to find another performer to take on the role of a puppet, or even a cartoon character, but like Hubert and others have pointed out, you can't just select, at random, a new performer to play Elmo, the performer has to be someone who is invested in Elmo, someone who understands the characters on all levels almost as much as Kevin built up those levels. This is also a different ballgame, where it might not be wise for the new performer to try and make the character his own, this isn't like when Marty took over for Snuffy and Telly and eventually made them his own as opposed to trying to immitate Jerry's Snuffy and Brian's Telly, or even when Kevin virtually reinvented Elmo altogether after he was passed by Brian, and eventually Richard.
Whatever the outcome be, we're in for quite a wait, but nevertheless, we'll still just have to wait and see.