I think this case has been made to Toyguru in the past. His argument is that in order to offer Castle Grayskull (even under a subscription or pre-order model) they would still have to create a prototype: the customers would want to see what they were getting for their $200-$300. As it stands, the line can't afford the resources to even make that prototype.
Personally, I wouldn't need to see a prototype. All I'd need to know is that it's a Castle Grayskull, and I'd already be opening my wallet. But I don't think I'm the typical fan in that regard.