A seniority system is used by virtually every airline in the US. Much of what’s posted about vacations ect.. regarding a seniority system is wrong. At most US airlines you can choose between pay and quality of life. You can use your seniority to bid a higher paying aircraft or stay on a smaller aircraft if you want to insure summer vacations and weekends off.
The important thing about a seniority system is it removes all pressure on pilots to make decisions management may favor but contrary to flight safety. The most important job as pilot has in today’s industry is deciding when to say no and shut a operation down or divert. Seniority systems give you the ability to do that without any fear of repercussions.