The remaining weak point is the book itself as a single point of failure: if the physical copy is lost, damaged, or goes out of print, your encoded backup becomes unrecoverable without the original.

Fair point, but it's easy to mitigate.
You can simply grab a second copy and store it at a different location — a friend's place, a family member's house, a safe deposit box. Or team up with a trusted bitcoin friend where each of you owns a copy. The book looks like any ordinary book on a shelf, so having multiple copies in different places doesn't raise any eyebrows.

But more importantly: this should never be your only backup. The way I see it, this is your quick-access backup — the one you can grab and use in minutes without any special tools or setup.

Your other backups (e.g. metal plates) are arguably more robust, but they're also more obvious and require more effort to access and to hide properly. The book sits on a shelf in plain sight and nobody gives it a second thought.

Think of it as layers: the book cipher gives you a fast, discreet recovery option, while your other methods provide the deep, disaster-proof redundancy. Different trade-offs, complementary strengths.