Every lent copy is a chance to have another reader eagerly waiting to buy my latest book. Remember that the same flap occurred over radio broadcasting of records. There's every reason to think that easy digital access to manuscripts would increase the sales of traditional books, as readers could try before they buy. Think of how many people buy videotapes and then don't watch them . They buy them because they enjoyed the movie and want to own it in their library.
People do the same things with books.
They buy a copy of a book because, having read a friend's copy, they want that book in their library. In short, I just don't think that publishing is going to be affected negatively by digital copying. That's why I used to make my manuscripts available online for free during the months between my writing the books and their publication. Especially the bit about stimulating the interest to buy. I hadn't thought of that.