I understand what you're getting at, but that doesn't mean that that artist's work is any less valuable. In your example, the artist in question is still being paid to create genuine art. I suppose there is an argument there that with ai available to create similar art, he may otherwise lose commissions to lesser quality substitutes. But on the other side of the coin, it's possible that many people emulating his work would accept a much lower quality substitute for convenience and wouldn't have paid him to create anyway, particularly given that d&d art is often used for conception and token purposes and not quality.

On the topic of ai art affecting that artist's reputation, I can see how it might become a problem that certain ai pieces are incorrectly attributed to an artist. That problem seems largely unavoidable though to people unwilling to differentiate or inquire as to who the artist is in the first place. That might also be something subjected to copyright law in the near future, like an ai water mark being required on any ai generator.

I still don't think that creative work is even going to come close to being trivialized. Ai art is free to the public, and yet I still know several artists who live off of commissions, one of which is my wife, because ai can't yet produce what people actually want to see with the refined quality that they can get from a real, human artist.