Chicago based United Airlines and Delta have become the most profitable U.S. airlines by targeting premium customers while also winning a significant share of budget travelers. That is squeezing smaller low-fare carriers like Spirit Airlines, which filed for bankruptcy protection on Monday. Some travel industry experts think Spirit’s troubles indicate less-wealthy passengers will have fewer choices and higher prices. Other discount airlines are on better financial footing but also are lagging far behind the full-service airlines when it comes to recovering from the COVID-19 pandemic. Most industry experts think Frontier and other so-called ultra-low-cost carriers will fill the vacuum if Spirit shrinks, and that there’s still plenty of competition to prevent prices from spiking.