DVC is a 50year contract, why would you view it as a 10yr or even 30 committment? Agree that many 'get their money's worth' by the 10 or 30yr mark, but you will still have to pay dues until the 50 yrs is up (or whatever your specific term is). DVC currently has a strong resale market due to ROFR, but that could change in the future, so it's risky to purchase assuming you can resell at a good price in 10yrs. We purchased with the expectation that we'd be using (or at least paying for) our points for 50 yrs. We are trying to ensure we are getting our money's worth in the first 10yrs, so that if we don't use it as much after that, or end up trying to resell and get low $$ for it we won't be disappointed.
There is some risk to any kind of projection looking 50 years out. Honestly, even 10 years out. You really have to bake that in. There is no such thing as a free lunch, and your up front purchase is buying that risk in exchange for part of your cost savings.
You actually can derive a purely objective financial value for DVC ownership by modeling costs and savings over a period of time and discounting to present value, etc. It doesn't really matter what that value is for my point, which is that while you can't know for sure what your DVC membership will sell for in 10 years, you do know that you will still have at least 2 decades of DVC benefits remaining (and more for longer-term contracts), so there will be at a minimum that value. This assumes that Disney and DVC are still going concerns and attractive destinations 10 years from now, which is something that I view as an exceedingly low risk proposition.
As far as other determinants of value, well, we're in a pretty much crap economy right now. Nobody can promise you what it will look like precisely in 10 years, but historically we are sitting in the middle of a very low point, so looking again from a risk probability perspective, while possible, it is relatively unlikely to be worse in 10 years.