It's funny that you mention supermarkets, because the A&P grocery store chain was from 1915 to 1975 the largest grocery retailer in the US. At its peak in the 1940s, A&P captured 10% of total US grocery spend. It too came under scrutiny and was accused of being a "monopoly", as its cheap prices forced smaller competitors out of business. And yet despite being a "monopoly", it began to decline in the 1950s when it failed to keep pace with competitors that opened larger supermarkets with more modern features demanded by customers. It continued its slow decline until it completely shuttered in 2015. A&P was once as recognizable as Google or Walmart is today, and yet most people have never even heard of A&P.

Sears too once had a "monopoly" on mail order products. People marveled at how amazing it was that you could order a product, mail in a check, and receive the product six weeks later, all without stepping into a store! Now they're bankrupt.

The whole concept of a business "controlling" the market is ridiculous. Amazon operates in a highly competitive, low margin industry that requires a tremendous amount of logistical overhead to function. The minute they stop innovating, they're toast.