No they are not. Monopolies are legal. What is illegal if they use (abuse) their market power to keep competitors out. For example AT&T had for the better part

No they are not. Monopolies are legal. What is illegal if they use (abuse) their market power to keep competitors out. For example AT&T had for the better part of a CENTURY a monopoly in the US. That didn't trigger the forced breakup. Instead: > In the 1970s, the Federal Communications Commission suspected that the American Telephone & Telegraph Company was using monopoly profits from its Western Electric subsidiary to subsidize the costs of its network, which was contrary to U.S. antitrust law. https://en.wikipedia.org/wiki/United_States_v._AT%26T_Co https://en.wikipedia.org/wiki/United_States_v._AT%26T_Co.