An AGI with a superior intelligence could probably also design totally autonomous factories. Being smarter than us it could even convince us to help it in the beginning in that regard.
Regarding the post-AI-art, this still presupposes that humans will be somewhat superior, out of the AI creative league, despite it being more intelligent, and that the AI won't actively work against human interests -- something I wouldn't bet our existence on.
And secondly, you make the same mistake with those who say after automation people will have nothing to do. Incorrect, people will have discovered a million new things to do and get busy at them. Like, 90% of people used to be in agriculture and now just 2%, but we're ok.
When AI becomes better than us at what we call art now, we'll have already switched to post-AI-art, and it will be so great we won't weep for the old days. Maybe the focus will switch from creating to finding art, from performing to appreciating and developing a taste, from consuming to participating in art. We'll still do art.