Pretty much summarizes all the main points. The people arguing against you don't understand history at all and are stuck in a self-absorbed bubble of ignorance and shortsightedness.

Slavery ending in first world nations didn't magically reverse centuries of systemic damage and prejudices. The immediate effects of emancipation in the US were brutal and discrimination still factors into modern problems. Fuck, voting rights are still being hassled constantly with bullshit attempts at blocking voters by methods that clearly target disadvantaged groups over others.

You can have all the super magical powers you want. But unless you can literally rewrite society and people as a whole in an instant to undo all of those factors all at once, solving slavery in such settings cannot be a "snap your fingers and it's fixed" issue.

And from a writing standpoint, the intricacies of truly addressing a subject like that in a wish-fulfillment story is laughable at best. It would end up being the entire focus of the narrative. Which a writer could totally decide to do. But attempting it and doing it poorly would be worse than the tolerance it's given in most stories. At least in that way, the author is acknowledging that the issue is far too complicated to handle lightly.

Obviously slavery is bad and one of the most disgusting things to exist. But just saying "And MC-kun ended slavery in 30 pages" is a disservice to the people impacted, currently living, and those that have suffered from slavery.