One would think that since ebooks are digital, any number of people could borrow the same book at once from a library. Alas, that is not the case. Just like libraries might only have one or two physical copies of a book, they might only have one or two copies of an ebook available to lend as well. If you want to borrow a new release, you might end up on a lengthy wait list. Additionally, if you borrow from a library, you miss out on a ton of books (especially self-published books).
As a reader, I'm excited about the prospect of Kindle Unlimited. As an author who self-publishes on Amazon, I am apprehensive (though the compensation for my books that have been borrowed with Prime has seemed fair so far).