The acceptance of slavery was not universal. Back then there were already people, abolitionists, striving to end slavery. So looking at what small but vocal groups protest now can give a hint at what future people could find wrong or evil.

It's not at all arrogant to think future people will share my ethics, just like abolitionists were not arrogant to hope for a world without slavery. Arrogance would be believing they'd share all of your ethics, and just as well I'm sure most abolitionists were still very racist, sexist, homophobic, ...