How about the claim that the Diamond Mining company is a monopoly? I don’t remember the name of the industry, but I think Milton Friedman even used to say this was one example of true monopoly.
And a widespread common objection (and defense of anti-trust laws) is the fact that many industries have barriers to entry. That is, it takes an awful lot to whip up something to even compete with them so businesses won’t try. How would you guys answer that?