A cold-hearted calculation of the value of the output of a slave compared to his "salary" and his "costs of overhead" (which are considerable despite his miserable living conditions) finds that when you evolve into an advanced, mechanized agricultural economy, much less into an industrial one, the use of market labor is more profitable than the keeping of slaves.
For what times those "cold-hearted" calculations were made? I bet the times when fertility of the man has produced sufficient amount of units relatively to the number of units needed to support upper classes, fight wars, etc. Invention of "Manufacture" in England has made peasants less "efficient" than sheep grazing on formerly peasant's land. Peasants were crowded in the city's slums to fuel industrial revolution. To name that "fuel" anything but slaves with the right of a choice what bridge to die under is disingeneous. Human unit just lost its value for the masters of life. Isn't that simple supply/demand stuff, commodization of the human life? The whip of guard was generously substituted with a whip of fear of joblesness, etc. and a carrot of "rags to riches". A unit was generously allowed to guard itself (under close guidance).
In the slaveowning South, slaveowners hired free Irish to do dangerous work like roofing. Slaves were too expensive to risk. About living conditions, you've got that wrong again, Irish in the slums of Detroit were doing worse than Southern slaves circa 1850.
It's not slaves being expensive, it's free men being cheaper than slaves and easier to manage that killed slavery.
This is why slavery ended peacefully in every country in the Americas except Haiti and the USA. It just petered out due to attrition and then its demise was made official by the governments. The same thing would have happened in the Confederacy if we had allowed it to; they could not have kept that nostalgic medieval economic system running for one more generation.
I'll give you that. Lots of po whites in the South were as cheap/poor/desperate or cheaper than slaves. Sharecropping could commence before Civil war. Slavery was just too hard to disband. First, slaves were WAY more expensive than white poor slob. Nobody wanted to lose that value. Second, poor whites were given "wage of whiteness" to keep them pacified and separated from blacks. Freeing slaves could give them wrong ideas.
I have a bad news though. Each of us uses equivalent of 300 personal slaves, if warp drive will not be invented and oil will eventually run out, cold-hearted calculations may show that the use of slave labor is more profitable. IIt's all about supply/demand of the human commodity.
