> Let me ask it another way: assuming that we have a copyright system, why should my copyrights ever expire?
Because copyright exists for a reason, not by nature. Assume we have a copyright system, sure. We've got one. The thing is that people made that system. Why did they do it? What was the reasoning for making that system, and is the current system still functioning the way we wanted it to function when we created it?
We can't completely ignore the reason why copyright exists, because copyright isn't real. Copyright isn't a right, there's no natural moral reason to for it to exist, it's not a property right and there's no coherent framing of copyright as a property right that makes sense when examined with the same rigor that we use to examine other property rights. Copying an idea doesn't deprive anyone else of anything other than a monopoly over other people's behaviors, which is not something that we usually care about when talking about natural rights.
We don't claim that inventing a better mousetrap is violence because it puts the old mousetrap makers out of business, so there's no good way to phrase "a monopoly over profits" as a natural right that people are morally entitled to. And the more we go down that road (independent invention, the fact that copyright can't exist without a public domain because very little of what we create is original, etc, etc...) -- we just keep finding problems and situations where copyright doesn't make sense as a moral right.
But there's an even bigger problem: it's not just that copyright isn't a moral right, copyright is also a restriction on the rights of culture itself. One of the most human things in the world that you'll find in every culture is that human beings share things with each other: ideas, stories, thoughts, and emotions. Copyright restricts people's freedom to share ideas, to fix their problems, and to exercise their own autonomy. It also intrudes into the free market, allowing emerging monopolies over content, and allows content to be weaponized to manipulate other parts of the market. It also restricts the collective autonomy of a culture to define itself and and to define its own cultural language.
However, despite all of these problems copyright is also a very convenient way to make sure artists get paid for the things they make. This is why humans invented it.
So the question becomes: given that copyright is a pragmatic restriction of both individual and cultural rights, are we going just far enough to make sure that artists get paid and that they're incentivized to make new art, or are we going farther than that and restricting people unnecessarily?
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> The less loaded way to phrase what you’re saying is that without copyright, there’s nothing preventing someone from copying my work, not that it’s somehow inherently everyone’s property (which is literally to say that it’s no-one’s property.)
To be clear, I am not in the first camp. I would say that no one has a moral claim over an idea, you can not logically own a thought or a story, and I don't think that is a way of looking at the world that stands up to scrutiny. I would not say copyright is just the way that society protects your idea, I would say that an idea is literally "no-one's property", the latter part of your sentence is correct.
I'll kind of issue the same challenge back to you in the opposite direction: I have heard moral arguments for copyright based on ideals of fairness or out of people deserving compensation when they help society. Some of those make sense, that's stuff we care about. However, I have never heard an internally consistent explanation of how someone can inherently own an idea that stands up to scrutiny in the real world; IP as a natural property right doesn't make sense either from a Libertarian/Anarchist perspective or from a Collectivist/Communist perspective, it's kind of amazing how many different worldviews it clashes with.
There's a huge amount of work that has been put into the phrasing of copyright and IP as "property" and "right", the names aren't accidental. But it's just pure propaganda, it's phrased that way so it sounds better to you. Copyright is a purely pragmatic invention designed to encourage production and to compensate artists. It has the same moral weight as any other administrative law we've invented: that is to say important and beneficial and worth having around, but only important in so-far as it accomplishes its original goal. Past that point, IP is an abridgement of actual natural rights to things like free speech, cultural exchange, and even (when we get the DMCA involved) actual property rights to tamper with and inspect products that people actually own.
There are decent arguments for multiple different copyright policies in multiple directions. But most of the decent arguments are based around pragmatism and policy (whether or not copyright is actually accomplishing its goals in its current form); they're rarely based around some kind of mythological right people should have to control everyone else's ability to imitate them or share information freely.