Your slave labor will all come work for my sandwich shop, because I give them better wages. Or if there's more good labor than can work for my sandwich shop, s

Your slave labor will all come work for my sandwich shop, because I give them better wages. Or if there's more good labor than can work for my sandwich shop, some will open their own sandwich shops, which will treat their employees worse than me but better than you, and you will go out of business. What stops this from happening is usually land-use restrictions and regulation - if your employees can't rent storefronts, then they can't compete with you. When those have been relaxed, this is in fact what happens: all the folks dissatisfied with working for you start food trucks and you get a vibrant foodie scene.