The seniority system has advantages for airlines as it stifles the free movement of labour. If pilots could walk across the road and start flying in a week, then keeping them happy would be more important to management.
Or the airline would just take captains that accept lower salaries over the existing staff who might be militant. That is Bob works for x airline as captain, but b airline pays 15% more, so he takes 10% less than the current pilots to get through the door but still gets 5% more than his current job. He shafts several FOs out of their natural progression and the company then only offers the staff that rate from then on. It's also much easier to dupe an outsider about how good your pay and conditions are, by the time they pass the rosy glasses phase its too late and they have accepted the remuneration package. Thats what the Libs wanted to enable with individual contracts, and is what happens in reality as most will take what they can and not take up the fight alone without a group behind them, aka unions. Seniority might suck for movement, but it does protect us from those that would 'work for less' for no other reason than to fly something else.