That is quite a hefty bullet to bite: one can no longer say that South Africa is better society after the fall of Apartheid, and so on.

That's hardly the best example you could have picked since there are obvious metrics by which South Africa can be quantifiably called a worse society now -- e.g. crime statistics. South Africa has been called the "crime capital of the world" and the "rape capital of the world" only after the fall of the Apartheid.

That makes the lack of moral progress in South Africa a very easy bullet to bite - I'd use something like Nazi Germany vs modern Germany as an example instead.

In my experience, most people don't think moral progress involves changing reference frames, for precisely this reason. If they think about it at all, that is.