Seniority in an airline has nothing to do unions. If seniority was about unions then there would be a NATIONAL seniority list like there are for autoworkers, crane operators, merchant marine, electricians etc. Seniority is a corporate thing. All of the Guvs union bashing aside, it isn't the unions that are responsible.
There are dozens of non union airlines out there. They all go to a seniority system. Its FAR cheaper for the company to reward longevity via bidding and whatnot, than to actually give decent raises.
Airlines that did away with seniority or didn't use a seniority system had way more problems as a result. The airline that I used to work for had tried it for a spell. It simply didn't work.
The other thing is that we train to a standard, this isn't fighters where the best man survives and the loser dies. They either pass or don't. How would you sort it otherwise?
Seniority protects the company from all kinds of turmoil. In its application by the airline, it disadvantages the union, by reducing the ability of ALPA to help victims of layoffs or industrial action.
Cheers
Wino