[indubitable]

A reasonable litmus test for monopoly is whether they would fail if they started exploiting their customers.

Time Warner and Comcast pass this test. They refuse to compete and then aggressively raise prices, seemingly to no end, knowing full well their customers have no choice. Microsoft's Windows 10 also passes this test with extreme anti-customer behavior that relies exclusively on consumers tolerating only due to a lack of choice.

Now back to Amazon. If they started ramping up their prices they would fail, fail hard, and fail practically overnight. There's a difference between a company being incredibly efficient and passing, with great success, that efficiency onto consumers and monopolies. Another general trend in monopolies is that they generally rely on tertiary forces to prevent competition. For the telecoms, this is the lack of access to wiring or even simply poles. For Windows, it's native execution of decades of third party software as well as the vast majority of all consumer software written today. There's no barrier to competing with Amazon, you're just not going to do it as well as them. It's the same reason WalMarts destroy small local businesses.