There might be other factors to consider, such as seat density and maintenance costs. Plus that deregulation was a massive change compared to whether or not they can use the AirBnB price quote model.

The cost savings has also come from efficiency gains like winglets & jet bridges, and service reductions like going from meal service to snacks to nothing, the removal of amenities (remember playing cards and wing pins for kids?), little or no in-flight entertainment, etc.