Spirit Airlines (SAVE -10.60%), the discount airline that at one time had agreed to be acquired by Frontier Group Holdings (ULCC 11.10%) before jilting Frontier for another, is being forced to fly alone. Investors are rallying into Frontier shares on the news, sending the stock up 11% as of 3 p.m. ET.Frontier could end up the big winner even after losingFrontier's business plan was borrowed from Spirit, and when the two airlines announced plans to merge in February 2022 to create the nation's fifth-largest ...