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.
Put another way, it is the belief that the 2020s are the peak of manual human art.
Imagine if we'd felt that way a thousand years ago, or even a hundred. Is it worth being wrong?