The point of the slavery example is that all countries have decided slavery is bad, and fought to stamp it out. That humankind, after millennia of slavery as the way things are, has rejected slavery. This is an example of moral progress. You pointed out that some people are slavers, and this is a good point; despite moral progress, even moral progress for all of humanity, some people still choose to do things that are wrong: there is hope of exposing evil, and hope of fighting it alongside all the world's governments, but not so much hope of every human rejecting evil, unlike in the baboon story. Yet, Barry Cotter says, this doesn't mean the fight is insincere - lip service and government passing silly laws like the drug war - it does reduce slavery. There may be little hope of every human rejecting slavery, yet humankind has in fact decided that slavery is wrong, and will free as many slaves as it can. Moral progress is a real force in the world.

That it can't necessarily help people faster than they are born, and thus may let total damage grow, is completely irrelevant.