It is more complicated than that; the Dutch settlers of much of South Africa settled there because the land wasn’t populated. Years later, people migrating down the southwest coast of Africa were entering their lands; the Dutch had to deal with them. It wasn’t slavery.
Not a defense of apartheid; just a clarification.
I know it wasn't slavery. They should have maintained their borders and evicted the migrants. We're encountering a similar situation with migrants from Latin America. A majority helot society will eventually be run by the helots. In ancient Greece, the helots were at least physically indistinguishable from the aristos (thereby making the identification of targets for revenge a little more difficult just from a casual look) and lacked this ideology that the aristos were the root of their problems for the rest of eternity.