In this case in particular, the relative seniority argument is only accurate with ZERO retirements.
The AT pilot group is younger and and the average tenure at any given seniority level is around half of that of an SWA pilot. In ten years, the top 10 % of the seniority list would have been disproportionally AT by a large percentage whilst having half the longevity of those just junior to them.
The folks who say longevity shouldn't matter are usually those with less of it.
Again, I'm not trying to start a fight, I am just pointing out that relative is not as "fair" as some believe. In the exact moment it occurs, maybe. Down the road, no way!