With more observers bracing for a future where JetBlue Airways Corp. and Spirit Airlines Inc. have to go it alone, Fitch analysts on Wednesday became the latest to cast doubt on Spirit’s prospects as a standalone airline, saying the ultra-low-cost carrier faced “serious headwinds” to improving its profits.The credit-rating firm made that assessment after a federal judge on Tuesday blocked JetBlue’s JBLU, -8.67% $3.8 billion bid for Spirit SAVE, -22.47%, arguing the proposed tie-up would stifle competition i ...