It's true that if no one archives it, it's gone. But there are people willing to archive things like this, and copyright makes it illegal to do so.
Edit: it's not the archiving that's illegal (probably)--it's the sharing of the archive that is. And sure they can share it after the copyright term runs out, but that's so far in the future the odds of an unmirrored archive surviving are slim.
Copyright does not make it illegal to archive. There should be a better exception about how to make accessible archives in case the source goes down I agree. But it should be nothing to do with copyright term.
Edit: it's not the archiving that's illegal (probably)--it's the sharing of the archive that is. And sure they can share it after the copyright term runs out, but that's so far in the future the odds of an unmirrored archive surviving are slim.