Spirit Airlines helped popularize the ultra-low-cost carrier (ULCC) model in the U.S., offering low fares while charging extra for services once included in ticket prices, such as checked bags and seat selection. This approach reshaped the airline industry and was eventually adopted by major carriers like American, Delta, and United, which began offering basic economy tickets with similar restrictions. Spirit's growth also brought it into direct competition with both major carriers and other discount airlines. Frontier Airlines, another ULCC, faces similar pressures but remains in slightly better financial shape, reporting modest profits in recent years. While Spirit and similar carriers helped transform the industry, their rapid growth, combined with rising costs and intensified competition, has made sustaining profitability increasingly difficult.