Why should we believe that unrestricted free speech results in the triumph of the... best? most truthful?... opinions?
Science, thank goodness, never adopted Mill's speculative view: science ruthlessly winnows out bad papers, bad experiments, and bad theories, giving them the least exposure possible -- perhaps just an editor, perhaps an editor and two referees -- so that only better science gets published. (Of course, mistakes are made! But the system largely works.)
So in morals, why should we expect free speech to produce good results? In fact, I claim that in morals have excellent reason to think it will produce degeneracy: after all, who doesn't like to be told that those vices they want to indulge in really are virtues? Does someone think "sex, drugs, and rock and roll" won out because it was the best advice, or because it panders to what we are inclined to do anyway?