The obvious counter “create new stores” runs into the next issue: Amazon could use its clout to prevent or limit creation of new competitors - there are many ways to do this (see MS monopoly case) - like requiring suppliers to prefer it over other parties, conditioning prices for some goods on discounts on others (“windows is the default/only install on all your machines or we will charge you a higher price for it”)
Then there’s using funds from one place you have significant dominance to to undercut (including running at a loss) in the new industry - I’m much iffier on exactly at what point this becomes uncompetitive as it seems a fairly common thing for many companies to do.
The important thing to note in the above is specifically pricing related, it’s about using market power to control the ability of competitors to actually compete (how can you compete if Amazon has ensure the best price you can ever get for any product is >= to the price they get)
Now it’s also important to realize antitrust regulation is not about producing the cheapest product for the end user/consumer, it’s about ensuring a fair market.
IIRC the Sherman act did not touch on consumer pricing, but rather producer prices: standard oil had a functional monopoly on the market so would tell people who found oil what they would get, regardless of the amount, value, or final sale price of the oil. So consumers may have been getting “cheaper” goods, but the people selling oil were getting screwed.
That’s where I’m going to guess antitrust is going to be targeting Amazon - just based all the stories over the last few years of them screwing third party sellers - for such sellers they _could_ sell directly (and many do) but if you aren’t on Amazon you essentially don’t exist, then Amazon has been found to duplicate successful products and sell them cheaper than the original product. I have no idea how prevalent that behavior is, but that sounds awfully like the kind of thing that would be subject to antitrust concerns.