In comic books the issue is not just long-lived or immortal characters like Wolverine, but in fact most characters seem not to age much, and in the rare event they do die they usually don’t stay dead. So the ‘life of the creator plus 70 years’ term is effectively eternal for most superheroes.
As to the bit about the Constitution, well, the Supreme Court considered what ‘limited Times’ means in Eldred v. Ashcroft, 537 U.S. 186 (2003). In a 7-2 decision the Court pretty firmly held that, so long as the copyright term is finite (i.e. literally not infinite), then Congress may set the term as it pleases, including extending the term of existing works retroactively.
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I don’t think even an apparently immortal character like Wolverine would upset the Court’s analysis because immortality isn’t really provable. Once someone dies you can say “yep, they were mortal alright,” but as long as someone is alive they are potentially immortal.
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