1) The day Amazon feel threatened they can become the cheapest in the market quickly again.
2) Given purchasing power scale and people like fast delivery there will only be a handful of companies that can complete with Amazon ever, so it seems unlikely there will be a 'started in the garage' business compete with them now the market is maturing.
3) With the extra profit Amazon are making vs a competitors they can develop and buy technologies or business to keep a moat around them for a long time.
I'm not saying they are irreplaceable. I do feel there is an inertia there that will be difficult to overcome combined with Amazons ability to change course given Bezos complete rule makes a fall vastly less likely than continued dominance over the short to medium term.