As harsh as it may sound - The US domestic airline network has a massive over capacity problem. The market on the whole can simply not justify and sustain the current number of seats
Overcapacity?
I don't think so. Maybe a couple of years ago.
Of course being intimatedly involved in the airline business I would never invest in it (Fuel fluctuations, SARS, Volcanic Ash disruptions, Unhappy labour, over and undercapacity at the wrong times etc. )