United Airlines and Spirit Airlines are clashing over the future of ultra-low-cost carrier (ULCC) models.  While Spirit has thrived on this model, it filed for its second bankruptcy in under a year late last month.Speaking at an industry conference on Thursday, United Airlines CEO Scott Kirby predicted Spirit Airlines would soon be out of business. "That is a fundamentally broken business model, and the consumer has voted. They are going out of business because customers do not like their product," Kirby to ...