I can't agree with On-MarkBob's comment.
As a military chap who is looking to join commercail aviation over the next 12 months, the seniority system fills me with dread. I'm used to working hard to achieve the next promotion and if I don't and I'm not good at my job, I won't get promoted. Makes sense to me! At 37, I am unlikely to find myself in the LHS of any major airline this side of 50; no matter how hard I work, how good a pilot I am or how flexible I remain.
Alternatively, the seniority system that most airlines observe rewards nothing but time done. Pretty much every other industry is capable of recognising hard work and success and yet the Airline Industy says that any other system would by un-managable. Why?
As has been said before, the seniority system is a very socialist approach to a very capitalist industry.