Demand also changes. Some author like Dan Brown is all the rage for a while. Ten years later, they may not need all those copies--maybe just keep the one or two copies of some title that are in the best shape. To greater or lesser degrees, libraries do review their circulation records. If a circulatable book hasn't been out for ten years it's probably a candidate for deletion unless it has some unique interest.

Sorry, I think I misread your original comment as implying that libraries were somehow overbuying, or replacing books that didn't need it.

I think I assumed you were using the term "core business" sarcastically... maybe I'm just used to so much snark on HN and the internet in general that now I'm seeing it when it's not there. :S

Never mind, sorry for the confusion!