I assume you’re talking about the bible, which is obviously not copyrighted, so where does the idea comes from that nobody will continue to buy old content if it’s in the public domain?
Translations of the Bible are almost always copyrighted, unless it's the King James version, which IIRC isn't really that popular anymore. For that matter, all translations are copyrighted.
The copyrighted translations are not thousands of years old, which kind of defeats the original point. I mean I understand it was a kind of exaggeration for effect, but excepting a very few 'evergreen' titles, the value of a publisher's back catalog tails off very rapidly with age.