A couple of points to consider and remember:

1) Once a flight takes off, any revenue lost from an unsold seat is lost forever.

2) Airlines WILL NOT stop giving miles on lower fares such as DL L/U's etc. They will instead increase the mile yield on higher priced fare classes.

3) While not the most scientific of surveys, look at airline capacities. Even at the absolute PEAK of the economy the best airlines were not doing better than 89% of capacity filled or so. That means that 11% of seats were EMPTY, adding in another 6-7% of seats given towards FF programmes and it is appx 82%. There is plenty of room to give "free" seats.

4)Most of the public has very little idea of runs and the like and is very limited idea of how to reap maximum benefits. Look around you! How many people do you know that are really savvy on this stuff?? Personally many of my colleagues travel like hell and have no idea about most stuff and don't care about it.