A big problem with the ULCCs in general is that there were so many young pilots hired initially. For those that join today, they only way that they will have a decent career with good progression is for the airline to continue to grow. For those that start at a Legacy today, they will move up 1/3 of the list in 5 or so years, and even if they are 30 years old, will retire in the top 300 of 15,000. That is top 2%, and will retire making over $400/hour (or a lot more as inflation continues to grow). You simply can not get things like that at a legacy unless continued growth happens.

This year, the legacies and large major airlines combined will be hiring about 1000 pilots per month. It won't take much to hire large chunks off the bottom of the seniority lists at the ULCCs, with a few senior pilots mixed in. Hiring to cover attrition only means stagnation on the list.