Slavery should be valid in all eras... maybe not optimal, but perfectly valid. I see no reason it wouldn't be and the argument that there wouldn't be factories if there were slaves is (and I say this with respect) absolute BS. It would just make factories cheaper to run and get more productivity out of the slaves you have is all it would do.
If we look at history, there are certainly examples of industrialized countries having slavery, but I don't think there is an example of a slaveholding civilization industrialising. At least for some time slavery must be gone, or there will be absolutely no incentive to build factories.
Just by looking at history, even science should take a hit. It should not come as a surprise that the massive increase in technology only started after slavery (and serfdom, which is pretty much slavery without slave markets) was gone. Labor
must be fairly expensive to start thinking about labor-saving mechanisms,
and there are more people who are allowed to get an education, which means that less prodigies are wasted doing menial work. In that sense,
every step that is taken to give equal rights to more people should be a benefit to all kinds of commerce and to production. If an Albert Einstein had lived in medieval times, say, as a woman or a serf, we would simply have had an immensly smart person doing back-breaking labor all the day. The same is true for any other scientist, engineer, artist, or any other kind of "Great Person".
Simply put, in a "hyper-totalitarian" state where no one but the dictator is allowed to have good ideas, the nation has the skill-set of one person available, whereas in a free state, where everyone is basically on an equal footing (at least with respect to the law), the set of skills at the nation's disposal is much larger. Even if we disregard all ethics, here is perhaps the main advantage of a free country with equal rights for all.