So here I am playing as Siam. I manage to abolish serfdom and then also abolish slavery time passes. It's been over 50 years with no slavery, yet the Landowners still have the Pro-Slavery Ideology. There should be an event or journal entry that removes this Ideology after X years of not having slavery. With 50 years of no slavery, there shouldn't be all that many people left alive who owned slaves themselves. It's time to let go.