I understand and agree with all this. But then your point isn't that you can't save money, or nobody saves money, etc. with
DVC. But rather, it is to understand that there are risks involved, and understanding those risks and how you could manage them if these scenarios arise. This is also true for other parts of the assumptions, though, and it is just as important. For example, some want to assume guaranteed 8% returns on your money that you set aside for cash reservations instead of buying DVC. Over the very long term, it is not unreasonable to believe that can be achieved. But annual fluctuations are also going to be normal for that kind of return, and also the point at which you invest the money makes all the difference in the world. If you are regularly making contributions to savings then you mitigate this risk because you will be buying at multiple time and price points. But if you are looking at the alternative to buying DVC points, that is investing your money at a set point in time, and if that is at a relative high point in the market, your returns could be far, far worse even in a rising market. It is pretty standard, fundamental advice to not invest money that you need for near-term use in volatile instruments, which includes stocks and stock mutual funds. So this isn't necessarily the best assumption to use for the return on your "vacation fund", unless you also understand that risk that a big market drop could wipe that out as well.
The point is, there are risks no matter what you do. It is a mistake to believe that DVC is a risk-free opportunity as you astutely point out, but it is also a mistake to assume that about alternatives that will bring you 8% (and even less than that) annual returns as some (not you) also assume.