I agree it makes it less likely that people will create novel manual art techniques. By analogy -- when cameras were invented, artists stopped focusing as much on realism. But I don't think that was any great loss. Also, there are some artists today who still make photorealistic paintings, despite the existence of cameras; so I don't think Stable Diffusion will make it impossible to have advances in manual art techniques.

Also, I think we have to weigh that tradeoff against all the new things that will be created if art becomes accessible to 100x as many people. On net, I think Stable Diffusion opens many more doors for art than it closes.