There's no question that the resale restrictions have cost
DVC sales from a certain type of informed buyer. The question is whether those same rules have allowed DVC to add sales in other areas. Specifically, has the ability to say "you cannot stay at all 15 resorts unless you buy direct from us" become a net positive for sales?
There seems to be a perception that the direct purchase restriction is universally bad for sales. DVC doesn't benefit from enacting policies which blindly punish those who buy resale and drive down aftermarket values. Informed buyers are likely to be more sensitive to resale pricing. Relatively uneducated buyers who tour and consider buying while at WDW will only see value in the idea that a direct purchase is necessary to maximize program flexibility.
*IF* the policy is working as intended, there are more people choosing to buy direct because of the resort access + perks than there are people who run scared from the impact on resale value. And if it's not returning the benefits intended, DVC can always change course.