Are you objecting to the word itself, to the underlying contention that slavery is a bad thing, or to the meta-level suggestion that including certain elements in stories is more likely to lead to problems and bad writing than to useful story ideas?
There are certain things that do disgust me when they appear in stories, but overall, I don't mind if characters do things that I morally disapprove of. For instance, I believe, as the Christian tradition teaches, that homosexuality is inherently immoral. That said, I can still enjoy a story that has main characters that behave in a homosexual way. Similarly, we all agree that slavery is bad, yet slavery as a story element can be used correctly. Additionally, there have been many stories of slaves falling in love with their masters and becoming free through marriage.
The problem is that, as was stated earlier, the slavery thing is just used an excuse to have the hot chick be with the loser who just so happens to treat her correctly. I would much rather have a story where the guy earns the slave girl's affection through something other than "being nice" (as you can be a complete sociopath and "be nice"). Have him slay entire monsters to save her life, have him give her a priceless treasure, or, hell, win her over with a nice dinner and conversation about a hobby they share. IMHO, "being nice" doesn't really earn you much than a pat on the back. "Congrats, you weren't a complete shit to this obviously sympathetic character. Whoop-dee-do."
Do you get what I'm saying?
That doesn't happen in shallow WN, because it's like Mary Sues where people either completely fall in love with them or are morons condemmed for not liking the MC at best.
If anything, as with all harems, everyone wanting to have sex with the OP protagonist doesn't really break my suspension of disbelief. It's broken when said women want to have sex with the MC because they're all legitimately completely in love with and devoted to the MC with no motivation from material gain or any other form of self interest.
Exactly. Hell, I can see a harem forming around a guy because he's superficially handsome and badass. Boom, instant harem. Of course, the challenge of the story could be is to actually make honest friends with the girls.
But we're just getting sidetracked by the harem/waifu aspect of Isekai, eh?
Here's an idea I just had.
The Marvelous Land of Oz had of the characters, a orphan boy named Tip, turn out to be Ozma, the Fairy Princess. Ozma was transformed into a boy as a child and was raised as such in order to hide her from some greater evil. That could be the excellent starting point of an Isekai. The story could be an inverse of the Tales of Wedding Rings manga. Our brave young protagonist was raised as a normal young man living in Japan until their sixteenth birthday, where a wizard retrieved them and returned him to his true form (much to his chagrin) and must use some great magic to save the kingdom. What do you think?