Does Amazon's Monopoly Really Matter?

Yes. If you want to sell anything online (and actually have a chance it will sell) your choices are eBay and Amazon. Amazon treats its sellers like total garbage and doesn't even allow you to keep the customers that you have serviced (in many cases it can be years).

They also directly compete with third-party sellers by using marketplace data (so he third-party seller is essentially giving all data to the competition just by staying in business).

Seller support is a joke. If you get banned by their bots, which happens by mistake constantly, you are banned for life..and nobody will even talk to you.