The problem Spirit has right now is that the legacies win in virtually every category — even *gasp* QOL (except for AA… but their new contract looks to address that).
Seniority movement, upgrade times, total compensation (work rules, soft pay, and retirement), and so on. But as alluded to by other posters, Spirit is giving people virtually zero security that this company is making investments in its people and/or the operation to see through that they grow and exist 20-30 years from now.
Frankly, I almost hope Spirit gets acquired at this point (and god no, not by Frontier) just so we can secure our future.