We've already seen Kermit appearances on shows and skits, but Cookie would be something new! That's the draw. Plus, you're forgetting that this is a Sesame Workshop attempt and not a Disney one. If not Cookie, then what other Sesame character would you want to host? I think he's the safest Sesame choice due to his uninhibited nature. Personally, I support the idea of Cookie Monster very much and hope that he gets this one-in-a-million shot at hosting SNL. I'd also expect a few odd cameos here and there.
Oh Frogboy, as a live long Muppet/Sesame/all things Henson fan like you, just on that level itd be a crazy dream come true. You saw how excited the audience got, the gasps and claps when Kermit, Fozzie and Animal showed up in late 2004. And as a fan of SNL, I love unorthodoxed guests.
Cookie's appearances on both Colbert Report and the online show Rocket Boom proved he can be edgy, topical, funny in his own innocent way. But how dynamic could he be for a 90 minute show? With Kermit, I could see him dressing in drag, doing celebrity impersonations, even poking fun at the perception of puppets and the Muppets.
Obviously, I cannot think of other adult Sesame characters (adult as in meant to be grownups, like The Count and not kids like Elmo, Zoe, Big Bird, etc) that would be perfect besides Cookie. And definitely NOT as iconic and well known.