I think the reality is what it always was - those people who hustle, get the hours, get the four year degree, get the early upgrades and LCA positions and the like while avoiding DUIs, training busts, etc., AND THEN ACTIVELY APPLY TO MAJORS, are always going to be chosen first.
Those who wait for the flow are going to be last to get hired and will get the seniority that goes with that hire date.
Flow is a BACKUP, that’s all it has ever been, and perhaps an opportunity for those who would otherwise never have been competitive to get to a major. If the loss of senior regional pilots at AA regionals becomes significant enough to threaten the AA regional feed model, then AA will either increase pay at the regional sufficient to offset that or simply bring the regional flying back to mainline.
I doubt seriously they will pi$$ off competitive people from the 135 world, from other regionals, or from the military, by awarding seniority to people at the regionals, nor do I think APA would even allow them to do that if they wanted to.