I don't think Elmo and Zoe are the only characters kids in the real world identify with. But that's the impression the toy companies seem to have.
Actually, I've seen a lot more of the older characters recently, The Hallmark stores have Ernie, Bert, Snuffy, Oscar and Grover. In other stores and Amazon, I've occasionally seen Guy Smiley, Honker, Martian, Sherlock Hemlock and other dolls (which REALLY shocked me!) Imagine if there had ever been a Don Music doll! WARNING: NOT FOR HEAD BANGING!
To me it looks like there was a bit of a backlash after all the Elmo stuff.
However, most of the books and toys seem pretty much required to have Elmo on them. It just doesn't bring up warm feelings to me, when it looks like he NEEDS to be there. If you want the toy, you need to tolerate him, there's no choice in the matter. Not very democratic, Sesame Street! Heh