What qualifies as a field in which the alleged monopolist is the sole producer?
If one defines a field as narrowly as possible, then every producer is a monopolist because each makes a slightly different product. If one defines a field as broadly as possible, then every product competes with every other because people place all products on a single scale of values and choose some at expense of others. In the former view, a McDonalds' Big Mac doesn't compete with a Burger King Whopper. In the latter, it competes with Microsoft Windows. One can identify that Microsoft has a monopoly on Microsoft Windows, almost has a monopoly on operating systems, and isn't even close to having a monopoly on software, but one can't objectively decide which of these identification should determine Microsoft's legal status.
If one wants to, one can define every producer or no producer as a monopoly.