Carlson already has enough $ to retire, and he certainly would have nothing to gain from such a "spectacle for the masses". His place as #1 in the world (and arguably, history) is not threatened by Hans.
Fischer refused to play Karpov in 1975 and lost his World Champion title by default. Granted, Niemann is not Karpov, and no one is Fischer, and the situation was entirely different, but this is known as counter-example. The salient point is that if Carlson has a serious challenge to his title, he couldn't refuse to play and still keep his title.
