Inspiration is absolutely a thing. When Constable and Cezanne sat at their easels, a large part of their inspiration was Nature. When Picasso invented Cubism, he was reacting to tradition, not following it. There are also artists like Alfred Wallis, who are very unconnected to tradition.
I think your final sentence is actually trying to say that we have advances in tools, not inspiration, since the Lascaux caves are easily on a par with the Sistine Chapel if you allow for the technology? And that AI is simply a new tool? That may be, but does the artist using this new tool control which images it was trained on? Do they even know? Can they even know?
If you make a painting now, it wouldn't be based on those thousands and thousands of paintings since, although you have seen them, you apparently do not remember them. But, if you did, and you made a painting based on one, and did not acknowledge it, you would indeed be a bad artist.
The bad part about using the art of the past is not copying. The problem is plagiarism.