Bullshit anime logic. If the technology the MC uses is more effective and sustainable than slavery, then why ban it? If it's less effective, then it can't compete with the existing way they do stuff in the first place, so it's harmless.
Historically, slavery has been abolished all over the world, not really because we have advanced as a species, but because slavery is not effective in the face of the industrial revolution. Anyone in power will look at it and say, "Yeah, abolishing slavery seems like a profitable idea". Only a fool would cling to human slavery labor while standing in front of a gigantic, magical industrial production power.