Very, very few regulations are designed with the sole purpose of dealing with natural monopoly. Most are there to prevent some monstrous or dystopian consequence of unfettered capitalism that we as a society have decided is undesirable.
Having a tiny group of wealthy oligarchs own all media, press, acceptable thought in an area is one of those horrifying dystopian consequences that we apply these regulations to prevent.
Be careful any time you see an analysis of media consolidation that ignores the reality on the ground -- the media's unique ability to define and shape public opinion. Such an analysis is either so purely academic as to be not relevant to any actual political or social discussion, hopelessly naive or, worse, willfully ignorant in service of a particular ideology.