Empires, by definition, begin their decline at their peak. Today Amazon bestrides the publishing world like Caesar, and it may seem far-fetched to think of this company slipping from its dominant position. There is some doubt, however, that Amazon can continue to augment its control over so many facets of the industry. Although there may be more growth ahead, the environment Amazon operates in is evolving and rivals may force their way through cracks in the fortress.
Joe Esposito has a great essay over on Publishing Frontier. You should go read it!