" Part of what makes credit cards work is that they simplify transactions for both buyers and sellers. Concentrating on just a few cards further simplifies matters on both sides of the market. Thus ever since the big shakeout, no new credit cards have joined the ranks of the majors; the barrier to market entry has proved to be too great. That said, in recent years the Internet revolution has opened the door to competition from wholly new directions—including new kinds of payment services, such as PayPal; an international network of automatic teller machines to challenge old standbys such as traveler’s checks; and maybe even new types of “virtual money” such as Bitcoin. As I write this in 2014, Apple has announced a new payment system on the latest iPhones, and we can reasonably expect that it and/or other new payment systems that make use of mobile devices will become commonplace. "

― Alvin E. Roth , Who Gets What — and Why: The New Economics of Matchmaking and Market Design