I don't know, we have a post industrial world that largely shuns slavery: Yet today we have more slaves on earth than ever before.
There are lots of tasks even in a modern industrialized world that are done cheaper and even more efficiently by people than machines. Like, ah, sewing clothes, for example, picking grapes, strawberries and the like and a lot of other tasks...
And actually, industrialization with assembly line manufacture was taking a lot of jobs away from specialists and employed instead people that needed little actual knowledge of what they do, because it's just one repetitive task. Tasks salves are quite suitable for.