If amazon had real competition maybe they would not have raised seller fees so much, and would not have prevented sellers from allowing lower prices in other places. So a cases can be made that they are using their market share to drive up prices, not lower them. Costing the consumer more.
There's other issues besides cost, for example counterfeits. If they had real competition then maybe they'd have to do something about all the counterfeit products they sell, which hurts both buyers and sellers.
Just a couple examples. There are other ways that monopolies can impact markets.
I've had similar thoughts about facebook. We could have had much better social/messaging systems, but facebook bought the competition. We likely missed out on more variety and perhaps much better options. Consumers were harmed by these lost opportunities.
You would be getting the same products for cheaper, that's the point. Their "two days or less" hasn't been true for several years. "Prime" to me basically has become "sometime in the next week" and there's a massive warehouse within 30 miles of my house.
Capitalism is one tool in the free market toolbox, and it works very well in many ways. One way this tool doesn't work well is that it leads to monopolies, which stifle competition, which hurts the people listed above.