Monopolies are generally not sustainable without barriers to entry. In many cases these barriers are Government ownership and patents, copyrights, and legal barriers. None of these are created by the free market but rather intervention by the State.

Monopolies, generally, are unsustainable in the absence of intervention. This is because they lack the agility to adapt to changing market conditions, technology, and new competition. Anything someone is doing well at, someone else will be motivated to do it better. They also tend to become inefficient as their structures tend towards bureaucracy (just one example of diseconomies of scale). Government intervention tends to prolong this inefficiency.

It will be interesting to see how Amazon adapt their business model moving forward. I believe they will need to be open to accepting cryptocurrency.

I will be covering monopolies in my market structure series when I get back from Alaska.