When I consider a timeshare purchase, I am usually looking at it only for personal use, and expect to want to use it for at least the next 15-20 years. I assume that the only thing I will get is what exists in the contract at the moment I buy it. Future resorts or non-contractual perks aren't part of the value equation, because they might or might not be available to me tomorrow, let alone five years from now. I also do not assume that there will be any residual value when I am ready to dispose of it--I assume it will be worth roughly zero when that time comes. If there is some value left, great! That's gravy. Even if there is some nominal value, it will have been subject inflationary loss plus the cost of capital between now and then, so it's easiest to just ignore it.
In other words, I expect to get nothing other than the ability to stay in the resort(s) contractually guaranteed to me, subject to availability and my capacity to plan ahead. Plus, every penny I put into it is spent money that is gone forever. If, after all that, the value still seems to be there, then I buy it.
This is admittedly a conservative approach--very conservative. But, it has two major advantages. One: I am never disappointed by future changes in perks, access, or resale value; only pleasantly surprised. Two: it makes it harder to justify a luxury purchase that isn't paid for out of money I genuinely will never miss. Not impossible mind you, but harder.
