With sunk or fixed costs of operation largely unchanging no matter what the business model, a model that sells seats below their marginal cost is destined to fail eventually....
Or find ways to squeeze ancillary revenue from the customer's pocket.
The Achilles heel of the 'low fare airline' is that unit cost reductions are finite. Therefore they either grow yield, which the consistently struggle to do or simply make sure everything that was once included costs an additional am amount.