Obviously the price cannot continue to go up forever. How much could you sell DVC points in 2040?
When will prices start to decline as it nears the end. Who knows. My guess is that when it gets down to about 25 years high prices won't be looking so attractive.
Of course, as other threads the last couple of years have suggested, Disney may decide at some time that new resorts would need longer contracts, thus a DVC II resort plan. That of course is pure speculation, but it does go to illustrate that any number of things could happen in the next 10-15 years that could dramatically affect pricing.
Meanwhile, with 40 more years to go, I'm not going to worry about it now.