@lightng I feel like this is a troll post so I'll bite, say my piece and be done with it.
I think the fact that the fandom in Japan aren't trying to enslave each other is proof enough that they know the difference between reality and fiction. It is fantasy, fantasy meant to be unrealistic. That's the point. A slave trade doesn't have to reflect the ones that exist in real life, weapons do not need to function as they would in real life, and so on and so forth.
It also does not and I repeat, DOES NOT, need to celebrate anything. Murder is wrong, I don't agree with it, yet I'll happily watch Guts mutilate some poor unfortunate souls like no tomorrow. The stories are set in a world where none of what you care about matters, you read it to immerse yourself into it and not the reverse. If you find something you dislike in a story then either soldier on or head off to a different story. Fantasy is not made to be someone's moral compass, if that were the case then Dark Fantasy and all it's various genres would create the most psychopathic murder junkies you'd ever seen.
If you need a story to tell you slavery is bad in order to understand that it's bad then you're already beyond the cusp of normal human logic. Even comedians like Bill Burr point out that spousal abusers don't need to be told "Hitting your wife is bad" because they're not going to see that and think "Oh man, what really? I thought it was perfectly normal to tenderize the face of your loved one! Ah no! I'm so sorry sweetie!"
They know what they're doing and they don't need a tv special, a video game or any other piece of media to tell them that. Similarly, if someone agrees with slavery, they're not going to disagree with it simply because a story tells them to. Racists still exist even when ninety-percent of the population vehemently disagrees with it.
Also don't ask people to comment on the positives of something and then tell them to keep it to themselves, that shows immaturity and a blatant bias.
P.S. Also an argument isn't there to provide proof. PROOF is there to provide proof, an argument is made so that one has the logical reasoning necessary to believe something. Proof is there to show that that something holds true. Telling someone who reads a bunch of mermaid fiction that they'll drown if they try to breathe underwater because they're not equipped with gills is an argument, dunking their head into a filled bathtub is proof.
You don't need one to have the other in order for the premise to hold true.
Likewise, you can't prove that I am even human and not an advanced, experimental artificial intelligence. However, you believe that fact without any proof behind it what so ever because it's just the most logical thing you can think of. Good day.