AMZN will not falter, not the way it's going right now. Love him or hate him, Bezos is a genius and as long as he's in charge, share price will keep rising (although they really could use a split).
Amazon, as a company, is the prime example of adaptation, domination and evolution. Remember they started as a online retailer selling books, then once they got too good at that they started selling everything. Now they are getting too good at that too so they expand into digital and AI. And God know what else they will do once they dominate that market too.
Imagine a company that will saturate its market because it just keep pushing into new ones. That's Amazon, and that's what justifies its sky valuation.