There is absolutely no evidence of this at all. Every DVC property is built for that expressed purpose. As a result, all of these resorts add room options for cash customers. Attendance has increased at WDW, so there is no reason to believe that cash bookings have dropped. If you are concluding that people who would normally book at a resort are now DVC members, then how do explain that resort bookings have not dropped?
Folks, the benefits of DVC to WDW are huge. It is a way for the company to bankroll the building of new resorts with a quick cash injection into the property. If each resort attracts 8,000 new members, and each new member pays 15k to get in, that is $120 mil. And that does not include the popular add-ons that current members would buy. After that, the members then pay 600-700 a year regardless of whether or not they attend. Also, DVC resorts have much smaller housekeepings staffs, so they are cheaper to run. As far as member services goes, well it really can't compare to what is needed to take ressies for resorts. Member services is a much smaller operation.
So what does the cash customer get? Spend a night in a BWV or SSF, and you'll find out.