Everybody has to make their own decision, but your last sentence seems a bit foolish. If you think there is even a 1% chance you would leave, then you should be updating your apps yesterday. It costs NOTHING (or basically nothing) to do so and requires no committment on your part at all to stay or go. Then, when a different airline gives you a call for the interview, you should absolutely go through that process, again it cost you NOTHING (or basically nothing) and requires no committment on your part to stay or go.

Then when you get a class date you will finally know when you have to make a decision. If at that point you want to stay at Spirit, then you will finally be at a point where you have to make that decision.

I think your assumptions of the risks in your first paragraph are also a bit inflated on the legacy side and a bit of polyanna on the ULCC side, but again everybody has to make that assesment for themselves. What I can say for sure is the longer you wait the more likely it that folks that have lesser qualtifcations than you are going to be offered the job at the legacy.

So, my recommendation is for anyone to apply as soon as they can. Keep trying to get every door to open and don't close any until you absolutely have to. Right now the hiring environment has created a perfect situation for all pilots. The legacy "doors" have never been so open in our history, all one needs to do is apply!!! If you end up getting offered through the door, you don't have to go through. Just doesn't make any sense to not try to get it opened now and doesn't make any sense to close it until you actually have to.