Numerous times in history, the courts have issues consent decrees requiring large companies to break up into smaller competing companies for violating antitrust laws. The two best-known examples are American Telephone and Telegraph (AT&T) in the 1980s and Microsoft 20 years later. (AT&T was broken up into the "Baby Bells"; but the Microsoft breakup was successfully appealed, and the breakup never occurred.)
Many argue that breaking up a monopoly is a Pareto-efficient change. This interpretation cannot be so because breaking up a monopoly makes its owners (or shareholders) worse off. Do you agree or disagree?