Whoa here people! Everyone is talking about miles depreciating, but I think TrojanHorse hit on the real value of miles in his post. The value of a mile is linked intrinsically to the value of the underlying commodity it purchases. For example if a free coach domestic US ticket stays at 25K, then the value of a mile is appreciating at the same rate as the fare for this flight is increasing. For example if the fare for the flight you wish to take is $300 now and years from now at retirement it is $600, haven't the value of your miles appreciated as compared to the nominal dollar?

Now this is not to say that I do not think one incurs significant risk of airlines upping the reward levels, going bankrupt, etc., but if the program would stay the same it seems that the purchasing power of a mile is going up as airfares increase over time.