Im less bothered about the slavery. (It does fit in with a medieval~ish society unfortunatly)
But more that the mc spouts some bs about 'ooh It's engrained and It's going to be chaos to free them', like that's not a fucking excuse to not want to end slavery. Especially the economic and political system that forces parents to sell their children. But he just shakes it off and leaves.
Now, if he instead showed sympathy while realising that It's not something that he can do anything about as one person? That i would be fine with, it could have served as a reminder that this is not a nice fantasy world of swords and magic.
Because if legal slaves are criminals, like that pickpocketing brat, and kids sold into slavery, i doubt that they would have many rights. Since they are not considered people as the slave merchant showed.
Lastly, why no debt-slaves? Unless they become criminals by not repaying, i would expect them to make up a large portion of slaves in this kind of system.