Aircraft rent and insurance are only about 10% of our CASM while fuel is 50% of our CASM. I will give you at least a 60% advantage is aircraft rent and insurance. I would rather have a 30% advantage on 50% of my costs than a 60% advantage on 10% of my costs.
At low fuel prices, cheaper to own/operate gas guzzlers make sense. At higher fuel prices, more expensive to own/operate fuel conservative airplanes make sense.
Allegiant is a totally different business model. They don't fly into any markets with direct competition. As an example, the day Airtran started their Newburgh-Orlando Service was the last day that Allegiant flew the route. They new they couldn't compete with us (and later Jetblue who also entered the market).