This librarian
http://homepage.univie.ac.at/horst.p...e-library.html suggests that the average lifespan of books at her(?) library is about 35 loans, and that appears to include academic books. I wouldn't be surprised if it is shorter for novels. (Novels are read all the way through more often than academic books are.) People are not nice to books they don't own. They get tossed around, shoved in bags, and generally spend more time out and about than a privately owned book that lives on a bookshelf.
Note that I am not saying that I support this cap, just that 26 loans may actually be reasonably close to how long current hardbacks last. (Current hardbacks being more poorly made and less durable than their predecessors.) Can anyone find some real data?