As much as we would love to have every book in the library, there simply isn't the space. So we do occasionally "weed" the collection to make room for new titles and collections.
It's something we do a lot (all libraries do), and we work carefully when deciding when it is time to remove a book from the collection. Typically, books that we remove have not been used in a while, are in rough shape, have outdated information, or we can find at another library.