@Xoldin
There aren't necessarily that many isekai MCs that somehow become rulers of a region or people, so there's a lot less possibilities for them to affect the world large-scale and thus affect the slave system.
That being said, he didn't break down the system and free slaves, he just chose not to implement in his region due to poor efficiency. It's a whole different thing from another isekai MC conquering a region and then freeing all the slaves because they dislike slavery.
That being said, i'm curious if the succubi were already living nearby and came due to hearing about him, or if he for some reason summoned / created them.