I mean there are large monster e-commerce giants in other nations, it is a very real possibility that Amazon will have to battle with some company at scale who is also in the razor sharp margin mindframe.
Its not impossible to imagine a world where amazon gets into a war with an Asian e-commerce giant for domination of the worlds e-commerce needs. If this does come to fruition this battle will probably not be resolved until 2020 ish time frame... so its a real possibility that amazon could be in business for 30 years, never return a profit to its investors, and NOT end up ever ruling ecommerce.