It's my general belief that when a book goes out of print and new copies are unavailable, the book should fall into the public domain.
If the publisher is no longer selling the book, then the publisher and author are no longer receiving income from it. There is really nothing left to protect, no benefit to the public in protecting a work that is unavailable. Don't want your book to fall into the PD? Make an ebook available perpetually.
(No, it's not practical. But it's nice to dream.)