Obtaining a monopoly via legal means is irrelevant if there is then monopolization, the examples given in this thread of "product tying" via Amazon Prime, essential facilities denial via superseding with their own products, and predatory pricing via not having to pay platform fees are all restraining trade.
Whether these could be sufficiently proved is a whole other matter.
> the examples given in this thread of "product tying" via Amazon Prime, essential facilities denial via superseding with their own products, and predatory pricing via not having to pay platform fees are all restraining trade.
If you don't trade on Amazon's platform, you're not affected by any of these. You might as well complain about Safeway's (or whatever grocery chain operates in your area) anti-competitive practices, because Safeway will also do product tying via membership card, rewards and coupons, deny you facilities to put your products on their shelves, and won't pay carrying fees for its own store brand products.
Sure, it might be much harder for you to compete with Amazon if you can't use its platform, but then the argument is that the Amazon is too competitive, not anti-competitive, and that is in fact legal (and a boon for customers).
Whether these could be sufficiently proved is a whole other matter.