I completely agree, however, it is still a useful metric. And when a stock has a P/E of 910 the growth rate that the underlying business must grow at in order to justify such a price is astounding.
If capital expenditures are reduced just a bit, or if margins are improved slightly, the E part of the fraction will jump and P/E will fall massively.
All of that being said, there are still plenty of things that could go wrong for AMZN.