Tim, at some point they won't be able to build anymore.
There are always new customers for DVC. In another decade, people who are 15-20 year old high school and college students today will be married and taking their families to WDW for the first time. But there is some question of realistic expectations for sales pace and price. That's the equilibrium DVC will be challenged with finding in coming years.
If you compare to 2-3 years ago, DVC appears to be settling for smaller sales numbers while raising their profit margins. That's a legitimate approach to take, particularly when the product is one that can only be sold one time in 50 years. In a few months when all of those BLT points are gone, DVC may have to wait 48 years before they can start marketing another resort at the Contemporary.
Aulani and other off sites will certainly garner attention. As stand alone projects, those ventures won't be as profitable as a WDW or DL resort where Disney already owns the land. But if adding destinations like Hawaii, Washington DC and others help to grow the overall appeal of the program, they can still be justified to the powers that be.
Then they will have to focus on existing customer satisfaction. Part of the DVC equation can't just be the huge influx of cash from the sale, but the continuing locked in vacations to WDW. If they can't keep up the quality of the experience and vacation homes they promised then it will collapse in on itself.
If you're talking about quality of accommodations, I'm not particularly worried about that because DVC is spending our money. Quality will vary over time, as it certainly has in the last 10 years or so. But at the end of the day, DVC isn't going to run the resorts like a Motel 6 because we are the ones signing the checks.
My comments were directed more toward perks and extras. Owners are committed to using the points. If there isn't a steady stream of new business to maintain, there's really no reason to offer AP discounts, holiday mixers, Welcome Home Wednesdays, cruise gatherings or most of the other perks typically afforded to members.