Amazon's strength lies in their logistics/3PL/last-mile systems. They invested heavily into this very early on into their business and it's been paying off ever since.

Walmart is the only other company that can compete at that scale given their very mature ground distribution network and even then Amazon has them beat good on air cargo (Amazon has a virtual cargo airline operated by Air Transport/Atlas Air, both of whom they have some ownership over) and inventory management (they hired some very very smart folks to help them build inventory forecasting systems)

As long as Amazon remains the only company that you can buy almost anything and get it in two or three days almost anywhere on Earth, they won't need to worry about the grim reaper coming for them.