We know that that has been true in many cases, but we don't know that it always will be true in all cases. For example, I don't think there's a very good argument that competitive pressure has led to a super high quality healthcare system in the US vs. the rest of the industrialized world. Competitive pressure optimizes for certain things, and those things are not always what is best for the public. Especially, if "competitive pressure" is applied by a trust, it can cause market failures.
I don't know that Amazon is that. Maybe it will be. But, last I checked, we do still have ground floor retail. I think there's an equilibrium that we'll reach at some point, where retail space commands lower rents and has to provide a high quality shopping experience. The Best Buys of the world are done for, but the Nordstroms are not.