I agree those examples represent positive changes. But what is the direction of causation? Did people become less homophobic because "gay" stopped being used as an insult, or the other way round? Did nazism end because people stopped saying "useless mouths", or the other way round?

I think the simple answer is: it's both. Changes in attitude cause language changes, and vice versa. But the balance of these forces isn't necessarily equal, and it may tip from one side to the other at different times in history. I would speculate that, in a society with a sustainable pattern of moral 'growth', more importance is placed on improving our ideas (through open discussion, taking in a wide range of perspectives, including those we find disagreeable) and allowing most language changes to flow organically from that. If the focus shifts too far towards trying to directly influence what people say (at the level of words/phrases), we risk stunting moral progress by encoding the status quo as dogma. Which, unless we are sure we already have the answers to all future moral questions, would be bad.