Krueger essentially summed it up.
Getting rid of the seniority system would be foolish for a lot of reasons.
First and foremost it would be a detriment to safety. Airline captains today ( at least in the U.S.) can make decisions in regards to fuel, diverts, fatigue, emergency's, calling in sick, refusing trips, kicking off pax etc etc.. with out fear of repercussions with regards to promotions, pay or job loss. This is a gigantic benefit.
I have been with a large U.S legacy carrier for 15 years. I probably won't make captain for another 5 at my base. But based on my seniority I know I will one day, if I can pass upgrade training and if the airline is still around. It doesn't require that I have a perfect attendance record or that I play golf with the boss or kiss every captains ass that I fly with. I don't know the name of my chief pilot and I don't care to know. As far as I know he doesn't know mine which is fine with me. All this promotes a sense of security that would be absent with a non seniority based system. Does it mean that it's a perfect system? obviously not but it is by far and away better than the alternative.
I think if we could go back in time ALPA probably should have gone with a master seniority list when things got started in the 20's and 30's but it's too late for that now. Overall it is a good system and I think it would be absolutely moronic to change it.