Afaik, that's the (modern, post-1918, [0]) definition of a monopoly under Sherman Antitrust Act (of 1890).
And specifically, that a monopoly's nature and actions "[must] suppress or even destroy competition." Think duck typing.
Unfortunately, US vs ATT was never decided, as ATT volunteered to be broken up in 1982.
But the broader approach seems to have been that small monopolies are not harmful to the public good, as they face the potential of competitors (sometimes, larger ones) expanding from unrelated fields into their business.
It's only when a successful company leverages its success into a size (and dominance) that makes competition impossible that it becomes a legally actionable monopoly (by restricting even the possibility of competition).
And specifically, that a monopoly's nature and actions "[must] suppress or even destroy competition." Think duck typing.
Unfortunately, US vs ATT was never decided, as ATT volunteered to be broken up in 1982.
But the broader approach seems to have been that small monopolies are not harmful to the public good, as they face the potential of competitors (sometimes, larger ones) expanding from unrelated fields into their business.
It's only when a successful company leverages its success into a size (and dominance) that makes competition impossible that it becomes a legally actionable monopoly (by restricting even the possibility of competition).
[0] https://en.m.wikipedia.org/wiki/Chicago_Board_of_Trade_v._Un...