You make two really good points -- once these resorts are fully sold out, it is going to be harder to get the resort of choice and it will be quite worthwhile to own in the resort where you want to stay.
The longer "life" of SSR is a significant selling point, too. I believe that DVC increased SSR's "life" largely to make it a more attractive sell to existing DVC members.
I don't, however, think it is worth 31% more, because a year's points 50 year's from now is not worth a year's points today. One way to illustrate this would be to give you a choice of two different 50 year memberships, one running from 2004 until 2054. The other choice would run from 2008 - 2058. Most would pick the contract starting right away.
Ok, what if I offered instead a contract running from 2008 - 2063. I just threw in an extra 5 years!
I would venture most people would still pick the contract running right away, simply because they don't value those distant years as much as they value the earlier years. Even those taking that deal would not likely pay 10% more, even though they get 10% more years.
I agree that the extra years have significant value, but I don't think it is 31%. But, hey, I'm no economist! At $79, I agree that SSR is a decent buy for someone who prefers that resort to the others.