By Doyinsola Oladipo, Allison Lampert and David Shepardson WASHINGTON (Reuters) - A war of words between United Airlines and ailing Spirit Airlines executives escalated on Tuesday after the Chicago-based airline's chief questioned the bankrupt discounter's business model and expressed doubts if it could stay in the industry. Minutes later, Spirit responded. In a post on X, the Florida-based carrier said its customers love low fares and its premium product offerings. "Maybe that's why United executives ca ...