Library fines are not known to increase rates of item return and in reality are a commonly-cited reason why library users never return at all.

A few other points: 1) Libraries replace items all the time for all kinds of reasons, especially if the title is in demand, so a book walking away with a patron who wants to keep it isn't really a business-stopping problem. 2) Patrons can always ask library staff to request an items they need from another library; the library is likely to accommodate, especially if the item is long overdue and they don't have immediate plans to replace it. 3) If the item is somehow absolutely irreplaceable, it probably isn't circulating anyway.