While it's certainly true that contracts will approach $0 value as they end, I'm still not convinced that extending is wise from a financial perspective.
If you took that $15 per point and instead invested it with an average return of just 6%, in 10 years you would have over $26 per point. In 15 years you would have over $35 per point. Will a '57 contract be worth $35 more than a '42 contract 15 years from now? I have my doubts.
If your goal is to make money, I think you're better off investing the dollars elsewhere. IMO, you'd be much more likely to find a traditional investment vehicle with an average return of 7-8% than you will getting a similar return from a
DVC contract extension.