Suitcaseman,
A seniority list is the most equitable system that can be applied to pilots.
Firstly look at why this system evolved.
pilots would work on days off, minimise diversions, bust minima and work into discretion ( without discretion ) to impress their flight ops bosses in order to get the next shout for a command.
This was quickly shown to have an adverse effect on flight safety so a system had to be created which would allay flight safety compromises.
A seniority system which is an age based system, and age based because it is absolutely arbitrary ie it is a criteria over which the individual had/has absolutely no control whatsoever, was then deemed to be the fairest system that could be applied.
Remember a seniority list only determines your turn for a command or a vacancy. Meritocracy then applies as to whether you get the vacancy or not.
So a Captain leaves one airline and has to go to the bottom of a seniority list when he moves to another airline. Absolutely fair.
Why should length of service of the junior fo in the second company not be recognised? Why should a pilot with command seniority in one airline automatically be entitled to command seniority in another airline.
Many people who do not understand seniority do not understand that it does not preclude a meritocracy system at the same time.