Plenty of union contracts emphasize seniority when it comes to pay/benefits/promotions etc, which makes it harder both for younger employees to get pay increases and promotions out of turn and for people to job hop for career progression.

That’s a real concern in terms of meritocracy, but I don’t see how it makes your goals orthogonal to everyone else’s, unless you are always a young out-performer. Almost by default, if you’re not job hopping (presumably to get a raise) you won’t always be one of the younger employees. The out-performance piece is by definition an edge case.

The original comment seemed to lay claim that each individual will sacrifice for the collective to become a net negative. I think there’s a good case to be against seniority rules, but it’s almost impossible to claim they are against every individual self interest.