I believe this statement is too strong, and would like to disagree with its implication. It is of course true that miles do not accrue interest. But their underlying value is the cost of a flight (or benefit) and that increases in value with inflation. I suspect that in the near future, airline prices will increase faster than inflation. (My prediction could be wrong, but you have to admit that it is a possibility.) In such a world, the value of the miles has increased with the price of the ticket that you don't have to buy.

A counter argument is that redemption levels go up. A counter-counter argument is that the cost of a saver-type domestic award on many airlines is the same as it has always been.