Are premium passengers the answer to budget airlines’ problems?

Targeting premium customers has proven profitable for Delta and United Airlines. 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...