Are premium passengers the answer to budget airlines’ problems?

Delta and United have become the most profitable U.S. airlines by targeting premium customers while also winning back a significant share of travelers on a tight budget.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 that travelers on a budget will be left with fewer choices and higher prices.Other discount airlines are on much better financial footing than Spirit,...