Political correctness doesn't exist because it has no meaning.
Political correctness has no fixed meaning. It is a 'catch-all' phrase used almost entirely at the discretion of the user.
Because it has no fixed meaning, when someone says something is "politically incorrect" or someone else dismisses something as "political correctness in overdrive" all they are really saying is, "I don't believe that is a legitimate opinion, however, I don't want to challenge it." For example, if someone disagrees with those that believe the Washington Redskins is an offensive name for a sports team, they may, as Del Jackson Miller did, dismiss their opinions as "political correctness on overdrive." In referring to their arguments in this way, Miller is dismissing them offhand without engaging with them. Political correctness, as a stand-alone idea, does not exist. It is just another rhetorical tool both sides use to shut down opposing opinions without challenging them in open debate.
Just because it doesn't have a fixed meaning doesn't mean it doesn't exist. If society is nothing more than a group of intersecting oppressors and victims, then political correctness exists in the human interaction that takes place at these intersections. To say political correctness doesn't exist is to fail to see the way our language contains subtle a deep-rooted prejudices that perpetuate oppression and marginalisation. In this sense, political correctness is not just there to dismiss opponents ideas out of hand, it is there to perform a social justice function and create a more balanced society.
[P1] Political correctness doesn't exist. [P2] Therefore, as a concept, it is neither beneficial nor detrimental to our society.
[Rejecting P1] Political correctness does exist.