IMO, inevitably the resale prices will decrease. Resales will hold their value as long as the buyer can get enough vacations in to break even on the initial purchase. The closer to the end of the contract, the less time to vacation and reach that break even point. On the flip side, as resort costs go up so does the cost of non-DVC vacations, requiring fewer vacations to reach break even.
Of course other factors affect the break even point: how the points are used (weekday vrs. weekends), maintenance fees, the value associated with the better accommodations available through DVC, demand to visit WDW, etc. If in 2027 it costs $14,000 for a 200 pt BWV contract, but resort costs increase to the point where itll cost at least that much for an equal number of vacations over the 15 years left, there will still be demand from buyers.
For me, its not about the resale value of my purchase, Id add on points as long as I feel I can break even before the contract expires. Once I buy a contract that money is gone, Im just going to enjoy the vacations and not worry if Im getting a good value by taking cruises, staying at resorts like Poly, or staying outside of DVC once in a while. Like the song Spending Money says:
I got no financial conscience.
Can't worry where it went.
A lasting treasure or a moment of pleasure
Worth it every cent.
If the vacations you get from DVC bring joy to you and your family, you made a great investment.