I worked in a library for a long time. The only way a book got thrown out is if it's condition was really bad, ie it was puked on or had a significant number of pages torn out. A book could potentionally be sent into storage but it was never thrown away for space, even books that we had ~200 copies of due to their popularity. And those books remained in the system and could be pulled out of storage if a patron wanted it. Financially, it's cheaper to keep it in storage and still let patrons check it out via book search than it would be to throw it away, and then spend money ordering it when a patron came in looking for it.
There were books on the shelf in the library I worked at that had been on the shelf, not checked out, for over 10 years. One book had the metal shelf end piece permanently outlined via the sun bleaching it into the cover.