Well I have no idea of what the future holds, but on a gut feel level I agree with the OP that my miles will bring less benefit in the future.

My reasoning goes along the lines that I believe that we've experienced the high point of air travel over the past 10-12 years. When I say that its from the perspective that the number of flights/airlines were increasing as were their routes. I can easily see a future that is much like when I was a child. Then it was rich people and business people that flew, everyone else took a flight on a "trip of a lifetime" and that was it (at least it was to a 10 year old from a not-so-well-off family in the 70's). And if/when environmental taxes are introduced to air travel that will be the tipping point (IMO).

I think the programs will remain, as the airlines generate a lot of revenue from selling miles. But as with other things they will need to find ways to cut the cost of using those miles and the easiest way to do that is to scale back the number of seats, and raise the price for the remaining seats. Some of this happens naturally as they eliminate routes. They'll try even more to cater to their remaining high-end frequent customers and that will leave fewer reward opportunities for the general public.

Hotel programs on the other hand I see growing, simply because of the increase in the human population and people's desire to "get away" on vacation. I just think that people will travel to fewer far away places than over the past 10-15 years. They'll take more trips via boats, busses, trains, and drives within a 5-6 hour radius of their homes rather than flying.