You are missing the point entirely. If you can make a movie with just a prompt, who is going to invest the money creating something like a Ghibli movie just to have it ripped off? Instead people will just rip off what has already been done and everything just stagnates.
The lower cost is not the bad thing. Allowing an AI to learn from it and regurgitate is the bad thing. If we can put anything into an AI and then say whatever it spits out is "clean", even though it is obviously imitating what it learned from, whoever puts the investment into trying something new becomes the sucker.
Also, I don't get this weird sense of entitlement people have over someone else's work. Just because it can be copied means it should belong to everyone?
Can you please explain how did you jump to this conclusion?
I fail to see how artistic expression would cease to be a thing and how people will stop liking novelty. And as long as those are a thing, original styles will also be a thing.
If anything, making the entry barriers lower would result in more original styles, as art is [at least] frequently an evolutionary process, where existing ideas meet novel ones and mix in interesting ways. And even for the entirely novel (from-scratch, if that's a thing) ideas will still keep appearing - if someone thinks of something, they're still free to express themselves, as it was always the case. I cannot think of why people would stop painting with brushes, fingers or anything else.
Art exists because of human nature. Nothing changes in this regard.
I'm sorry, but I do not think I understand the idea why and how Studio Ghibli is being "ripped off" in this scenario.
As I've said, art styles are not considered copyrightable. You say I'm missing the point but I fail to see why. I've used lack of copyright protection as a reality check, a verifiable fact that can be used to determine the current consensus on the matter. Based on this lack of legal protection, I'm concluding that the societies have considered it's not something that needs to be protected, and thus that there is no "ripping off" in replicating a successful style. I have no doubts there are plenty of people who would think otherwise (and e.g. say that current state of copyright is not optimal - which can be very true), but they need to argue about copyright protections not technological accessibility. The latter merely exposes the former (by drastically lowering the cost barriers), but is not the root issue.
I also have doubts about your prediction of stagnation, particularly because you seem to ignore the demand side. People want novelty and originality, it was always the case and always will be (or at least for as long as human nature doesn't change). Things will change for sure (they always do), but I don't think a stagnation is a realistic scenario.