[mc32]

There’s a parallel in beasts of burden. Do some smallholders use them? Yes. Are they economical? No. Mechanization undercut the value proposition of slave labor and beasts of burden due to the costs of upkeep and their nature of needing rest and recuperation. With slaves you also had to have a an infrastructure to keep them from escaping -also true for animals but keeping animals corralled is cheap.

[throwawaaarrgh]

Keeping slaves from escaping is simple: fear. You could be whipped or killed, you were unwelcome anywhere in society, and there were laws enforcing their capture and return. No infrastructure needed.

Slavery was wildly economically feasible, still is. All you need to do is add pressure to the workers and their performance increases. Like Amazon warehouse workers, but, you know, with whippings.

[mc32]

Fear wasn't the only tool --in China (the Qing Dynasty for example) it was socially ingrained that some people were destined to be slaves, this is probably the more successful approach, open to fewer questions or rebellions except when the lords overplay their hands and become cruel.