It's not an easy question, which is why the Supreme Court wasn't asked to decide it in this case. The answer depends on the past 100 years of previous cases interpreting US antitrust law. Those interpretations have been influenced by research and trends in economics. Lina Khan recently became a minor celebrity for her article in the Yale Law Journal critically reviewing this history in the context of today's tech giants, specifically Amazon: https://www.nytimes.com/2018/09/07/technology/monopoly-antit...