I imagine if the fees are not in, GenCon doesn't want to jump the gun and say no, WotC isn't coming, and I'm quite sure that in July, if WotC says, Hey, case cleared, we're good to go, got a place for us? there will magically /be/ a place.
If I were the powers that be at GenCon, which I am not, but if I were, I would have /2/ exhibitor floor plans, and I'd continue planning every single exhibitor up to Aug 13 with a place with, and without WotC. And I darn sure wouldn't make any announcement about their attendance until WotC confirms attendance.
In fact, I'd do that for any largish exhibitor. Heck, if Hasbro or someone says after the deadline, "Hey, we'd like X amount of space, can you fit us in?" I would say YES! Pay up! We'll absolutely work with you.
So I for one, am /not/ going to demand a 'final' list of Exhibitors. Doesn't mean I wouldn't turn down a list of confirmed, though!