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The supermarket where I work keeps all the floor people at minimum wage. Which is okay; I agreed to that deal as long as they taught me the business, which they're pretty much doing.
But for people who go there for their money to live on, what's the motivation to do good work? If the boss gives me the minimum, why shouldn't I give him the minimum?
If you won't pay good workers a bit more than bad workers, why would anyone be a good worker? How do you leverage or motivate the workers? You can't tell them that they're doing a bad job because so what? All you can do is keep them or fire them. And you can't fire them because any reason you can give for firing them won't stand up in court. You can't say that he works slowly or sloppily or he gives the customers the creeps. You can only fire somebody who does something dramatically wrong.
A strictly minimum-wage environment will, by itself, evolve into a population of exclusively marginal workers. And that goes for whatever the minimum wage is; changing it will have no effect on this principle.