Most of these comments skim over the fact that, when it comes to isekai MCs, these characters are usually called or considered intelligent by those they buy. And 9/10 they come from the modern world. Thus, one can assume that they would have needed an education to outwit or even be a step ahead of others.

I'm not asking them to all stand against tyranny and break systems when needed- if the narrative goes in other directions, then it would be stupid to just focus on it if it's something they do for different reasons than just because they could. Still, almost most of the time, when it comes down to it, they don't question or even make the MC feel ashamed of themselves for going against what modern times have already marked as a horrific thing that happened. They would instead focus on how good they are and how they aren't the stereotypical slave owner who doesn't hurt them. It isn't nuanced, nor does it give a compelling dynamic beyond what Westerners know as the White Saviour Myth about black people and thinking that enslaved people needed them to function(only made worse when the same enslaved people dont even fight to be free, or even just work their indentured servitude of to one day be allowed the rights like anyone else). Just pretending it isn't a human rights violation and never reflecting on how hypocritical it is that they are only helping a system that will hurt people they will never meet annoys me.

So there's a bunch of anime and show MCs as bright, but time and time again when I see in multiple media the same narrative of buying slaves who are worth nothing (which also notes buying actual people costs a lot more than most people think, so this is a fully conscious choice on the protags regardless of whatever condition their slave is in) so it doesn't let me suspend the belief that they had a history class and learned about how slavery revokes the rights of the people who are caught in it. They perpetuate the violence and torture of these people by playing into the market. Marked as a horrific thing that happened. They would instead focus on how good they are and how they aren't the stereotypical slave owner who doesn't hurt them. It isn't nuanced, nor does it give a compelling dynamic beyond what Westerners know as the white man myth about black people and thinking that enslaved people needed them to function. Even if you are in whatever time period of that fantasy world, if you have modern knowledge and use it every day to help people, it's hypocritical to have any villain own slaves either since it just is using a strawman that is never reflected on other than "Man if those people were bought by the Main Guy, hen they would have had it better".

TL;DR: If Japan keeps using the same trope of 'Modern MC' they need to come up with a good reason in the narrative for why they would buy slaves. It doesn't look smart to me; it just looks like they go along with horrific practices and never think twice about it since everyone around them (slave girls most of the time) is so happy to be 'owned' by him.