Actually, this is in fact what Zhao had imagined it to be, just like in the Black Wastes. Zhao would never send anyone to manage them. Rather, he would let the slaves build villages and let them restore their identity as normal civilians.
It seems like the Purcell clan also understand this concept. To get the horse run, you have to let the horse graze. Give the slaves a better life than other slaves, let them build their own families, and let them reproduce. In fact, even if the Purcell clan is a respected and big clan, it is impossible to buy several hundred thousand slaves each year to supplement the need for workforce. Instead then, they let the slaves increase their number by letting them get married and have children. Since those children are born from slave parents, these children are also slaves already after being born.