It's neither.
It wasn't active competition (or the lack of it, or it being reduced) that generated that outcome. It was Bezos (just as it was Sam Walton before him), he dictated that approach as the basis for Amazon's best likely ability to reach massive scale. There were no possible alternatives, if the goal was to get very large as a retailer. Amazon is Jeff Bezos, he has raised it like a child from day one, totally in control of what it is and what it became.
Bezos was facing a scenario of: approach it that way, or Walmart will take care of it for you eventually and make you redundant.
More precisely it was Bezos looking at the future and having to compete with a Walmart that fully awakened to online retailing. In that future forecast sense it was motivated by the threat / fear of competition that would inevitably crush a smaller Amazon. And if it wasn't Amazon or Walmart, someone would have done it most likely, Bezos surely understood that; that the Internet would make a mega platform like Amazon possible.
For another example of this premise in action: Microsoft wasn't so wildly aggressive and competitive due to the marketplace, or other competition or animal spirits in the water, it was solely due to the personality of Bill Gates. It mirrored his personality, aggressive nature and competitiveness. Gates ultimately made Microsoft in his image (that earlier version of Microsoft anyway). He dictated what it became, he controlled the growth and nature of its culture (and once you set that in order, it flows in that mold forward, a persistent mimic, until something breaks it).
Because it doesn't matter who ultimately made an anti-competitive world marketplace, it's still anti-competitive and ultimately impossible to run morally when you participate in it in parallel. eg if ebay was undercutting other people's auctions via buyout prices under their own brand
In that scenerio, and captialism in general, eBay doesn't owe sellers on its marketplace the right to sell items at all. They've just been given a chance to make hay while the sun shines.
I feel like a lot of people jump to attack Amazon on the basis of what they could do (because they're so big) instead of what they're doing currently.
Because - I believe - people are overwhelmingly reactionary, easy to excite and short-term focused. Microsoft was the end of the world. Google is the end of the world. IBM before them. Yesterday Walmart was the destroyer of worlds that was going to kill every small business in every town in America. Countless articles were written for a decade espousing that as fact, inevitable, unstoppable. They were pure evil, everyone knew it. Nobody could compete with them. That reactionary emotionalism against The Corporate Overlords, is now retrained on Amazon as the destroyer of worlds, the evil entity that will crush everyone. Tomorrow it will be someone else.
The most interesting story about Amazon - which is being almost universally ignored - is that the growth in their retail business has already mostly stopped (some shareholders are finally noticing). In the next few years that growth will continue to trend toward between zero and low single digits. The mighty retail conquerer of worlds is running out of retail growth and mostly that is being ignored because it doesn't bolster the story of the moment that Amazon is going to eat everything.
Those are the three things Amazon has done amazing at (lower prices, better customer service, new ways of doing things). Isn't that a paradox? You take the stance that only competition can breed this but in this case Amazon has done those things while still reducing competition. In fact, it is helping consumers that has led to people buying things on Amazon and it getting this big. Not the other way around.
Is it that people are afraid that at some point Amazon might stop doing these things? I agree that if what amazon is doing has lead to higher prices, worse customer service, and stagnant methods of doing things that would be bad. But thats not whats happening here.