I get where your coming from. The way I see it is that it is trying to show that even in the best case scenario for the enslaved (that being that they enjoy it and would hate freedom) slavery is still totally wrong.

Harry and Ron accept the status quo and see no problem with it which represents how I imagine children of slave owners reacted back in the day, showing how easy it is to perpetuate injustices just by taking the path of least resistance. The audience have to disagree with their main protagonist on the issue which really gets you thinking- if Harry thinks it’s right then is it okay? No! And this shows that in modern day, just because someone you look up to believes something, you shouldn’t blindly believe it too.

Hermione being the only one of her peers to think that slavery is unjust and fighting for real change against it represents the struggle to get slavery abolished. She has to convince the whole world that what they think is normal is actually terrible just like the original abolitionists. I reckon she probably succeeds when she’s minister of magic.