Apply this same argument to someone who spends hundreds of hours working so that they can buy a car. This is all done out of self interest. Most people would not conclude that it's okay for someone that a car thief, who steals the car because it serves his self interest. That this is somehow rational and moral because one person's self interest is equivalent to another person's self interest.
Of course people who are anti-copyright will be quick to point out that a bunch of bits on a disk that can be infinitely duplicated is different from a physical car. And that's true. But I'm not talking about that. I'm talking about the blood, sweat, and tears that the creator invests in the act of creation.
I could post more far fetched analogies. It's in a woman's interest to have self control over her body. It's in a rapists best interest to take control of her body. Does the argument that each individuals self-interest is reciprocal apply in this case? Is that the "sensible rational purely moral" conclusion? Of course not. There are a myriad of other factors that need to be taken into consideration.
Your car analogy conflates scarce and non-scarce goods. A better car analogy would be that someone invents a way to create cars for $200 with 3D printers, and so everyone stops buying cars that cost 100x that much. "You wouldn't download a car" and all that. Of course, someone has to create the CAD designs or whatever, but that's a much smaller industry than all the manufacturing that existed before (so, of course, all the people who work in auto factories are upset that they've been made irrelevant).
I explicitly acknowledge the difference between scarce and non-scare goods in the above post if you could bother to read the second paragraph. The part you're leaving out in your 'improved' anology is that it might cost 10 years and ten million dollars to develop this $200 car.
Basically the great-grand-parent is saying that there's no rational or moral justification for someone profiting from innovation, because it's in someone else's self interest to take that innovation, and these two parties self interests cancel each other out to the exclusion of all other factors. I find that to be an extremely naive and simplistic argument.
Of course people who are anti-copyright will be quick to point out that a bunch of bits on a disk that can be infinitely duplicated is different from a physical car. And that's true. But I'm not talking about that. I'm talking about the blood, sweat, and tears that the creator invests in the act of creation.
I could post more far fetched analogies. It's in a woman's interest to have self control over her body. It's in a rapists best interest to take control of her body. Does the argument that each individuals self-interest is reciprocal apply in this case? Is that the "sensible rational purely moral" conclusion? Of course not. There are a myriad of other factors that need to be taken into consideration.