I'm honestly not sure how you could make the case that they aren't maintaining and abusing monopoly power. I mean they literally blatantly copy the designs of other companies who sell on amazon, then put their own results for their own products higher in search results than what they copied off of. It's incredibly blatantly anti-competitive.
According to the best statistics I can find, in the US, Amazon has 37.8 percent of the retail e-commerce market. https://www.statista.com/statistics/274255/market-share-of-t... The simplest pro-Amazon case is that they do not have monopoly power, because they are not a monopoly.
It's also odd that their retail business doesn't make any money if they're supposed to have market power.