US aviation: is the ULCC model extinct in the US?

Recent moves by the US ultra-low cost carriers - Spirit Airlines entering Chapter 11 and Frontier Airlines' recent decision to introduce a first class product - beg one simple question: has the era of the pure-play ULCC model in the country come to an end?

The answer would appear to be: yes.

Questions are arising regarding Spirit's future as a going concern, and it is yet to be revealed whether Frontier's bet on first class will pay off.

Whatever the outcome of each scenario, the original ULCC model is no longer applicable in the US, as the rush to offer upscale products to bolster revenue could push Frontier, and possibly Spirit, into the 'value' airline' category, alongside JetBlue Airways and Alaska Airlines.