Frontier has recently expanded to fly out of Miami International Airport, which has been described as a big missing opportunity in the airline's route map. Frontier will offer lower prices than the legacy airlines that currently fly from MIA. This expansion should allow them to have additional momentum in the domestic flight arena.
Frontier Airlines is using one-day sales to gain people's trust, as well as win itself a reputation of being a marquee low-cost airline. For example, they've started a one-day sale of $15 fares from Chicago to major tourist destinations like Las Vegas and Miami. These unheard-of prices will allow them to corner the low-cost niche of the airline market.
While low-cost airlines are certainly doing well at the moment (including the reinvigorated Frontier Airlines), unexpected spikes in oil prices could abruptly end the party.
With ongoing wars in the middle-east already impacting commodity prices, it is easy to envision sharp spikes in the oil price. This would render the low-cost airlines a heavy blow to their tight budgets.
Frontier Airlines may just be peaking as the bottom falls out of their specific market.
Getting into the airline business is difficult and with Frontier focusing on affordable travel that will certainly help them grow in the future. I think it really depends on their margin though. If they can't sustain lower costs in the future than the fight now is virtually pointless. It's nice to see competition in the market.