I personally think Amazon's dominance in ecommerce of physical good is fragile and it would be hard to argue monopolistic behaviour there. If Shopify grows like it is growing right now, it would be quite easy for brands to build their own stores through it without bothering to list on Amazon. At least the ones with their own name pull can afford to do this.
Amazon hasn't really been able to use their dominance in online retail to any great extent. They failed or are after thoughts in every other market they entered. Video, music, phones, tablets, auctions, handmade (i.e. etsy competition) and their app store are the ones I can think of off the top of my head. Profit margins aren't great either.
The only really abusive thing that they do is around eBooks. They definitely have a monopoly there and doing anti-competitive things to maintain it.
Their AmazonBasics line has been very successful and incredibly anti-competitive too.
Before you go and compare it to brick and mortar in-house brands like Walmart has, consider that Walmart has a higher investment in things they end up stocking in store (they pay for things up-front and shelf space is comparatively scarce). Amazon doesn't have the concept of "shelf space", and Amazon doesn't paid for the product up-front in many cases.
Walmart is notoriously pushy on suppliers, but once product is on the shelf both Walmart and the supplier's interests are heavily aligned. Amazon would rather peddle you their Basics line instead.
I don’t think it’s fair, just like I don’t think Standard Oil could have pulled off a “We’re in competition with millions of people’s feet” and millions of horses.
Tangential thought that this reminded me of - I remember reading that when Rockefeller started Standard Oil, the total market for refined oil products was tiny, and the reason it was called "Standard Oil" was because it was so hard to find reliable refined oil products with consistent chemical qualities, and that the introduction of more standardized products contributed to the demand exploding. Within a few decades, the market for oil products was a few orders of magnitude larger than it was when Standard Oil began.
By 1890, even though Standard Oil sold like 90% of the oil in the US, the size of the market not controlled by Standard Oil was bigger than the total market when Standard Oil started (or close). So, in a sense, Standard Oil could have plausibly said they were in a competition with feet and horses? If they disappeared entirely, the market would have been the same size it was before they existed. Obviously the demand for oil would have gone up dramatically anyway without Standard Oil's anti-competitive practices, but it doesn't seem exactly right to treat Standard Oil as taking a slice of a fixed pie when the pie grew so much during it's existence...