Yeah, this is a very interesting dynamic. Doug Parker (and his right-hand man, Scott Kirby) have proven to be extremely successful competing against low fare carriers. They seem to have a carefully crafted strategy to drive them from their hubs -- or at least "persuade" them from expanding.

My guess is the strategy will work, because the only reason to fly Spirit/Frontier is if you really want to fly somewhere, but the cost is prohibitive on the "regular" airlines. If the savings aren't substantial, no one of sound mind would fly Spirit/Frontier.

I think Spirit/Frontier already operate at a huge disadvantage because they get almost none of the high paying business fares that "subsidize" the operation of the legacy carriers. So Spirit/Frontier already operate at a huge unit revenue disadvantage. Sure, they can operate at lower cost, but everyone has to buy airplanes and fuel. If the legacies prevent Spirit/Frontier from selling anything more than $49 fares, I don't see how they can cover their costs.