What if a very large group of artists, around the level of Disney, created art for the purpose of training and selling their own AI artist? Would DISNEY-E be ethical if 100.0% legal?
I suspect the real debate is the unfair advantage this puts in the hands of a few. Not copyright in the legal sense, but copyright in the labor sense.
Which is a perfectly valid argument, but that brings us to debating relative ethics over the absolute ethics of copyright violation.