I would question that narcissism is a mental illness in that it is a coping strategy that doesn't necessarily cause narcissists any discomfort or interfere with their ability to function, at least in their own self interests, in life. This is why narcissists usually have no interest in changing, because it's working for them.

Having said that, I also recognize that it develops, usually, in response to some form of neglect or abuse. In terms of pathology, I would hypothesize that narcissists could be seen as pathological to OTHERS more than to themselves.