People tend to forget that unions represent the existing employees, not aspiring hires. This can actually work to the detriment of juniors and newcomers to the industry. It can also work to the detriment of teams where it becomes intensely difficult to fire people who are legitimately underperforming. The public school unions keeping bad teachers on the payroll but keeping them idle in empty rooms is a prime example. Police unions defending bad cops is another very relevant example. If you’re an aspiring teacher or police officer with motivation to do well, why should those union employees take a spot on the job that you could otherwise occupy and do better? Switching the rules of the game to favor seniority doesn’t benefit people who rank low on the seniority ladder.