If you knew you could borrow it any time for free, and you do your reading digitally, would you really buy it? Would most people?
I personally have bought a handful of physical books from my favorites with no intention of opening them just to have them on my bookshelf, but there are also multiple series I listen to 2-3 times a year that I haven't purchased because they're either available through my library or because they're available through scribd (where I read more than enough new books per month to justify the subscription). Unrestricted free availability is bound to cut pretty heavily into people paying for content, especially if we're talking people who are doing their reading digitally with ebooks and audiobooks anyways.