If you look at a very broad sweep of cultures at various times... the classification of social classes get very blurry: slavery, peasantry, serfs, cottiers, indentured labour, wage labour... Which of these best represents Middle Kingdom Egypt's labour structure? Is sharecropping the same as medieval european peasantry?

The definition or labeling of these labour class structures don't tell you much about their economic implications. Slaves don't necessarily cost less than sharecroppers, serfs or tribesman. That doesn't mean it doesn't have implications, but they are complicated and relative to the specific of that system.

I just don't buy this linear extrapolation from A to B.