Then if you think you are worth that much, you can call their bluff and go somewhere else. And what happens in the union scenario? The manager hires an unproductive person for exactly the same wages?

Maybe you can, maybe you can't. If you don't want to support the people you work with then they have little motivation to support you in return if you find yourself treated poorly by the firm or a bad manager. I suggest you revisit the context in which the question was asked in the first place.