i expect them to remain the leader for at least a decade. the momentum is too large. i have thousands of orders on Amazon without a single truly horrible experience and there are many other sites on which i only ever had one order and it went truly horrible, like having to do credit card charge back. add to this Amazon's free same day delivery and 24/7 support that always picks up the phone and i rarely even bother checking other sites.
it would be an interesting thought experiment of what it would take to beat Amazon but honestly if i was an entrepreneur i would pick almost any other battle instead.