Explaining why slavery is wrong and why its proponents were monstrous is not the same as "lets have a continual public debate about my right to own you until the end of time".
Not only does this distract from productive intellectual discussion, it carries an implied threat that if a historically oppressed population ever slips up and fails to defend themselves that they'll be forced back into oppression.
Any explanation of why slavery is wrong falls flat unless you examine why it has been a part of so many human societies for most of history. You have to look at why so many people supported it, not just write them off as monsters.
Inevitably, that requires you to play devil's advocate. I agree, it's callous, insensitive, and unproductive if someone runs around telling other people that he has the right to own them. I don't think we should encourage that. But how can you understand a point of view if you're not allowed to state it?
That's exactly the ability we've lost as a society. The single most powerful thing you can do to bury the practice of slavery and everything like it is to sit someone down and tell them to write an essay explaining an argument in favor of slavery, then to defend it.
Slavery has been justified with many arguments. Some race based, some economic, some paternalistic. But what they all have in common is that when you explore them in depth and you see where they lead, you end up in a dark place. A place you realize you personally don't want to live in, that feels fundamentally wrong and detrimental to all the best things a human being can be.
That experience is personal, it's transformative and it's how a person develops a fundamental sense of justice which they then carry with them for the rest of their lives.
I think there is a lot of really bad discourse out there about controversial and sensitive issues (for instance, very little good comes out of having this type of discussion on Twitter, where everything is a reaction and people rarely think before they tweet). No that stuff is not productive, but there is a way to have these discussions which makes us better people. They used to be the subject matter of high school Civics until they were deemed too insensitive. We have moved backwards as a society by moving them to Twitter.
Not only does this distract from productive intellectual discussion, it carries an implied threat that if a historically oppressed population ever slips up and fails to defend themselves that they'll be forced back into oppression.