I think they have enough confidence in the product they are producing (people may disagree, of course!
) that they aren't too worried about it. Especially given the unique nature of it. I have seen a few people talking about canceling their 2nd or 3rd Grayskull (which I can't imagine ordering in the first place!) but not much in the way of people canceling their only Grayskull.
If there was some crazy uproar after SDCC and half the people cancel... it probably wouldn't matter. To us, anyway. The Castle would get made because it would be tooled by that point. The worst case scenario is Matty sitting on a bunch of Grayskulls they can't sell. But honestly the development of it would have to be catastrophic for something like that to happen. Even people who are grumbling about this or that about Grayskull are still going to get it because it's the only Grayskull Mattel is going to make.