"monopoly" is a red herring here. if they were under 10%, you might have a case (though there are many factors besides market share that need to be considered,

"monopoly" is a red herring here. if they were under 10%, you might have a case (though there are many factors besides market share that need to be considered, like supplier power). the issue that we the people, and by extension our antitrust regulators, care about is market distortion, and 15% is well within the range at which a competitor can unduly distort a market in their favor by sheer size alone. markets exist for the benefit of society as a whole, not just giant corporations.