I suspect -- but don't know for a fact -- that once a resort has sold a majority of its points, its difficult for Disney to offer Fixed Week deeds. Today, someone could walk into the
DVC Preview Center wanting to buy a Fixed Week deed for, say Week #49, at VGF. The problem is that right now, there may be no availability for a studio for the 7 nights that make up that week. If Disney can't fulfill the contractual obligation of giving the buyer the 7 nights to which they are entitled, then I wonder if it will sell the Fixed Week?
Of course, the buyer could offer to opt out of their Fixed Week for 2017, making that a provision of the sale. But I don't know whether buyers make that stipulation when they try to buy, or whether Disney would accept it.