Why are you assuming that each book gets scanned exactly one time and then never again? And why are you assuming that out-of-print books remain easily accessible so long as not every copy has been destroyed?
>Your local library throws out books every year and nobody thought twice about it.
When they're damaged beyond hope of repair from decades of wear. As for books that haven't fallen apart from being used as intended and merely are no longer desired, my local library generally sells them off at bargain prices. Of course I can't speak to your local library.