The doomsday scenario here is Amazon drives everyone else out of business, then profits from their monopoly.
But this monopoly would be on selling goods made by other people.
For them to be able to raise prices significantly without fostering competition, they would have to have abusive contracts with their supplies, like Apple tried to do with ebooks. That would be an abuse of monopoly power, and every potential competitor would be yelling for the DOJ to investigate.
Perhaps I'm missing something. How could Amazon significantly raise prices on their storefront, without abusive contracts with suppliers, and still block competition from becoming attractive?
Or are you thinking they would stick closer to the wal-mart model and keep prices low for consumers forever, and try to squeeze suppliers like Wal-mart does? If so, and that opens up a significant spread between the wholesale and retail prices, that still opens a window for competitors to undercut Amazon and make money.
I'm not going to try to list a huge number of ways that monopoly creates harmful advantages.
And monopoly is degree, not absolute, here.
As a company increases monopoly, it has any number of ways to milk the advantage, with the high cost of entry and ability to threaten competition protecting it.
During the period of increasing market share, it usually is a very attractive option.
Let's take the Wal-Mart situation. With the world-class distribution system they designed that relies on their massive volume, that's just one critical piece of competitive advantage.
There's a reason why so many businesses go out of business when Wal-Mart comes to town - and yet they can extract wealth from the community in massive amounts.
Some of that is 'legitimate value'. But when that power prevents competition, allowing monpolistic abuse, that's a problem.
It's a little like why the phone company had to be broken up - you couldn't just practically start a competitive phone servive with the advantages dominance gave AT&T.
At some point the advantages of massive size change from being efficiencies passed on to the consumer into protective devices to maintain monopolistic abuse.
Innovation tends to be just one of the casualties.