Ask a US Armed Forces officer (as in Lt, Capt, Major, etc.) about promotions based on "merit" in their system. As far as I understand, in their "up or out" system, f you are not promoted after 2 years of being eligible (with 2 opportunities for promotion) then that's it, you then retire. That goes for each rank. Wonder how much brown nosing, backstabbing, nepotism and politics goes on?
Imagine the chaos if the 20% of pilots who have their private, unfair, RP-ignoring arrangements with crew control turned into 100% of pilots trying to make private arrangements with crew control? And every manager. And every trainer.
The only people now wishing to get rid of seniority are the ones that have already benefited from it to get promoted but find themselves stuck at the bottom part of the captains list and can't change jobs to get better rosters while hating the non-truly-seniority-based issues that don't work well for them, like leave and type changes, or those who now wish they could take their qualifications elsewhere without starting over at the bottom.
It cuts both ways, and you can't have it both ways.