Air travel is a low margin commodity. The average citizen only cares about low ticket prices. If a mega-merger ever gets approved (not likely) this will be significantly detrimental to the traveling public. The gubment knows this.
It would not surprise me to see another smaller merger of two "super low-cost carriers" in the future, especially if oil prices go up. Beyond that, a mega-merger of the "Big 3" is not likely to happen IMHO.