The word "work" is being overloaded here, their work as in output might matter but I am asking why they must work at all in the first place. If your answer is because they must procure money to survive, that is an economic failure, not one of AI. Jobs are simply a roundabout way of distributing money for output to be produced, if an AI can produce the output, the job need not exist. This is the same argument that has been used for centuries as automation advances in every field, but suddenly, when it comes for my white collar high tech industry? It's an outrage.
Even then, their work as output can matter but that doesn't necessarily mean they (should) have a per se right to their work without other people also using it, especially in cases where their work is not used as outputs directly, which is what plagiarism is. If that were the case, no one could learn from a other's work, regardless of whether that one is a person or a computer.
Remember, we are discussing art here, not white collar tech jobs. AI coming for my job would be unpleasant and devastating, but that, like you said, is an economic problem. That I agree on.
I don't think there is a way to continue this particular branch of this argument without devolving into a debate on the value of human life like a couple of Macedonian philosophers - suffice to say, my point of view is that the work of others has intrinsic value tied to intent, and machines do not have intent.
If no output of humans has intrinsic value, then once machines can approximate humans sufficiently there is no reason for humans to exist - and that is an outcome that I, as a human, reject with all of my being.
Output of humans has value to humans; art does not have value to beings outside of humans, of course. That does not mean that one cannot use a machine to create new outputs, and it doesn't mean that those will or will not have value, as again, value is subjective to the (human) beholder. We see this already with people praising AI art. Therefore, I do not believe that intent matters in the slightest as long as people deem something valuable.
The reason for humans existing is not because of the output they produce (indeed, that is dystopic), humans have worth inherently, regardless of what they output. This is also what nihilists have figured out, so maybe that is something you should look into if you seriously have such an opinion as expressed in your last paragraph.
If you believe intent does not matter and is unrelated to human worth, then we are at an inherently impassible disagreement as to the nature of human society and will never agree on this issue. My belief is that this point of view, as well as others (like Nihilism, as mentioned) are fundamentally destructive to human society, which likely clarifies why you don't see a problem with what I and others believe is an existential threat to said society.
I don't see how your viewpoint is useful, though; Nihilism has been around for a century but I do not see how it is "an existential threat to said society." It seems like you believe something but don't have any empirical backing for it, therefore, let me remind you, as my other comment says [0], that people do not have the best record of stating why "society" is decaying.
Even then, their work as output can matter but that doesn't necessarily mean they (should) have a per se right to their work without other people also using it, especially in cases where their work is not used as outputs directly, which is what plagiarism is. If that were the case, no one could learn from a other's work, regardless of whether that one is a person or a computer.