That is indeed a convincing argument. However, it is an argument about modern humans. But what happens when you try to apply that argument to a sci-fi setting? I mean, in a sci-fi setting, you can get
all sorts of weird ethical problems you might have not even thought about. And because as a society we have not actually gone through the changes that come with the advancement of science or the changes in how the economy or the society or the government works (the steps through the discovery and/or adoption of a ground-breaking new thing and the adjustment of our moral system or the development of a new moral system around it) we tend to quickly apply our current morals to the situation without much thought about how the society in said fiction thinks and works or what the things in the story actually resemble, what their closest analogue to something in the modern world is.
So, to be specific, yes, there is a convincing argument that enslaving a human might be wrong, based on historical evidence. However, consider the argument from a different perspective. Is owning a work dog wrong? Dogs are intelligent. They can understand things, follow orders, make choices, display emotions... Yet many are imprisoned inside houses for people's entertainment while others are taken from a young age and trained to do something, without being given a choice. And if the training is right, they'll even learn to love their job and their trainer. But if they disobey their orders or do not behave as expected, they might end up cast out or worse killed, with a more suitable dog replacing them. The same argument could be made for any sort of domesticated animals, but dogs are usually the easiest to relate to.
So, we come down to two questions: What is the difference between a slave and a dog? I mean, if someone treated slaves like dogs or other work animals, kept them happy but forced them to do certain work, would what they are doing be morally questionable? And if so, why? What if they left dissidents leave and fend for themselves? Would they still be wrong?
And second and most important to our discussion, what is the difference between a Sod and a dog? After all, sods are intelligent but not as intelligent as humans, they were
bred created to follow orders, they are usually taken care of and they are happy as long as they are following orders. True, they have some human DNA in them and look humanoid, but so do other primates.
((Again, just arguing for arguments sake. I agree that slavery is wrong. But it's fun to see someone's morals and way of thinking.))