Economist Israel Kirzner insisted that monopoly analysis is only applicable when the possibility of competition is eliminated, either legally or by the producer's sole control of the total supply of a resource necessary for the production of a good or service, and that the term should therefore be defined as such. In which case Amazon is not a monopoly, and we shouldn't be surprised to see continually falling prices and increasing quality.
http://rationalargumentator.com/issue118/Kirzner3.html