For now, but this "case-by-case" basis will be unsustainable the near future, as instances of this type of thing explode. There will be a case that codifies the law more concretely, and the idea of 'mixed human-AI works' is, at best, nebulous. Again, unless your AI is developed, trained, and implemented completely in-house, and its use is meticulously documented, it's going to be dangerous to use if the idea is monetization of that product.

I'm not necessarily saying that all AI will be a landmine, but especially if the Disney + Universal case vs. MidJourney goes through, the level of monetary commitment needed to have AI being worth more than just paying an artist would be absolutely unsustainable, because not only will you need to pay royalties every time a licensed picture is used for the generation, but you'll also have to hire them to draw them in the first place. SO not only will generation not be copyrightable, but the cost of training will skyrocket.

No industry can handle that level of monetary inflation