What, precisely, does Disney have a monopoly on? Last I checked there were at least several major movie studios, TV stations, and animation houses doing very well for themselves independently of the mouse.

It does matter. Monopolies are not "good for capitalism" if they prevent honest competition from battling the monopoly for business. If, as argued above, monopolies naturally "self-destruct" in favor of newer, more ferocious competitors, then regulation isn't necessary.