The places where we've seen massive substitution are all industries where humans were in physical danger from the work being done. Risky manufacturing lines, etc.
That doesn't really reflect the situation. Machines have been substituted for humans since at least Eli Whitney's Cotton Gin. Substituting machines for humans has had nothing to do with providing safety, rather it has been driven by the desire for increased productivity and thereby profits. Certainly, this increased has provided massive benefit to society along with various drawbacks. That's literally the history of the "industrial revolution".