Airline cultures and QOL can change very quickly, especially during/after a merger. Just because Spirit may be a good place to be today in terms of QOL doesn't mean that it will be even tolerable a year from now.

If you are looking at airlines, look at longevity. Which airlines will still be in business in 20 years. Which airlines have the best hubs/route structure. Which airlines have the infrastructure to support the continued success of the airline and any future needs. Which airline will give you the best chance of having a long and successful career.

I would rather have a solid 30 year career with constant career progression as opposed to "It's a relaxed place to fly and has good crews", but then you have to start over again and again as airlines go out of business.

That is what is important.