THE trend seems clear. Spirit Airlines became the most profitable carrier in America at the same time that it was the most complained about. Frontier Airlines cut costs, became a no-frills budget carrier, and went from posting big losses to notching big gains. American, Delta and United scrambled to keep up, slashed comforts and amenities, and introduced their own no-frills budget fare class to compete with Spirit and Frontier, which turned out to be rather lucrative. The message was obvious: cut service, make money.