The seniority system being fair presumes the hiring system to be fair. Which off course it is not, as all the negatives having been listed in this thread (favoritism, etc.) apply to it. Hence you start out with a potential wrong, which the seniority system in turn rigidly will prevent from ever being corrected.
It also presumes that all its pilots will progress professionally at the same rate. If you were equals as newhires, you´ll still be equals in performance 25 years later. Right.
With a seniority list good work does not get rewarded and poor work does not get penalized.
Pilots are overwhelmingly right of center politically, but for some reason when it comes to this particular issue they turn into hard core socialist.
Ernest K. Gann in ¨Fate is the Hunter¨ (again): The seniority system must ever persist if only because it is a protection of the weak, who are everywhere in the greatest number.