AOL was not always the top online service. Before AOL existed, CompuServe was the major service, having bought out its closest rival, The Source. But CompuServe's domination did not prevent the upstart AOL from blanketing the country with computer disks containing its access software. Before long, AOL's membership surpassed CompuServe's. There is a critical lesson in that story.
Potential competition is every bit as important in an industry as actual competition.
Venture capital is always looking for new places to invest. If the dominant firm in an industry leaves room for a competitor to seize an advantage, someone will do it. But if the firm is so effective that no one wants to challenge it, consumers are the winners. The antitrust people still have not gotten the message. The only thing that can keep the process from working is the presence of legal barriers to the entry of new firms.