That's been true up to now, but even Ponzi schemes eventually show negative cash flow. At that point their originators shut them down one way or another. Are we there yet?
The airlines are still able to sell large volumes of miles (and are even attempting to sell substantial parts of FFPs) to serious, huge players on the financial markets. I take that to mean two things: 1) that the FFP schemes are generally far from over; and 2) that the present owners of the FFP are desperately seeking to raid the schemes' piggy banks (which they've successfully used in the past by not acting as a fiduciary for scheme participants) a few more times while hoping the "greater fool theory" doesn't bite them first. How many more times they do this and how much longer before they bite off more than they can handle is not yet clear, but it's getting closer and closer. [To carry on along the second point, this is also sign that the FFP operator insiders are no longer confident in the utility of FFPs to meet their needs as they once were -- and when management insider sales exceed management insider buying, that's a powerful signal to consider too.]
There will be buyers' remorse and there will be sellers' remorse, but not all buyers or sellers will be in the same boat. Alternatives exist when/where such alternatives are effectively sought by FFP members, and there are ways to play the game without being burned the way the FFP scheme operators might want the non-operator scheme participants to be burned.