This is the problem with “justifying” a law intellectually. Taking a clue from what Danny Sanchez elucidated in the Why Capitalism? thread, especially this paragraph…
Alternative legal codes won’t acquire near-universal respect just because people think, “well we’ve got to use some standard, and this standard is ‘salient’, so we might as well abide by that.” With regard to any given conflict of interests, there might be any number of “salient” standards one can choose, and individuals will always be inclined to support the salient standard that is pursuant to their own interest. Which brings us to the point that the only kind of saliency that can command near-universal assent to a rule is an ex ante tendency for the establishment of the rule to benefit any given person.
…we can see that Kinsella misses the point by calling 20 years arbitrary but calling forever consistent. Such framing puts libertarians on a wild-goose chase for the more “consistent” position, prompting suggestions like yours: “…what if the patent lasted until the inventor’s death?”
The supposed saliency, consistency, moral superiority, religious righteousness, fairness or nobility of any position always falls to self-interest. Fortunately Kinsella also makes a good case for the material benefits of abolishing IP laws, and that is where the force of his message lies.
Now I hasten to add that some of those considerations can still sometimes be a significant factor in self-interest. For example, if most people in society believe in a God who hates “theft of ideas” and punishes those who condone it, most people would believe it in their self-interest to uphold IP laws. Hence even without a state such a society would likely have some intellectual property restrictions in the customary legal order. Still, this would be balanced by the material benefits of idea sharing that Kinsella covers, if enough people were aware of them.
In the end, it all depends on what the people believe to be in their best interests (including psychic profit, afterlife, or whatever they care about); that will always determine what a society supports and what it condemns. If the people care about round numbers more than putting food on their plate, they might choose 10 years or forever or 0 years, but that premise stretches the imagination.