About a year ago, a Boeing 747 operated by Delta Air Lines DAL, took off from Atlanta for a three-hour flight to Pinal Airpark, a boneyard for unwanted aircraft in Arizona’s Sonoran Desert. The once celebrated giant of the sky, which had transformed international travel with its size and range, had flown its last flight for a U.S. airline. Delta has replaced its fleet of jumbo jets with Airbus A350s, one of a new breed of smaller, ultraefficient long-range airliners. Nearly every other airline in the world is doing a version of the same thing, replacing huge jets with smaller...