If you honestly believe that companies operate as meritocracies I have a bridge to sell you. Nepotism still runs rampant, attractive people get more job offers, promotions, and pay, networking still trounces experience and credentials when it comes to finding work, and credentials themselves are largely about wealth signaling.

Seniority is at least an objective measure, even if it is a terrible one. Besides which, no one says that's how a union has to work.

what meritocracy? are you saying that passing leetcode interviews means you know your job?

It's already the case in most companies.