

All of the research indicates that people on the far end of the narcissistic spectrum cannot be treated. NPD seems to be rooted in personality rather than it being a treatable illness. Trying to treat them would be like trying to make an intellectual person no longer curious about the world around them.
Post modernism seems to be overly concerned with the human condition being like wet clay. Moldable into anything. But evidence suggests that some people can be selfish, sadistic and disagreeable because of evolution. No amount of therapy can make them less selfish. Pharnaceutical research suggests that some drugs can very briefly allow narcissists to experience empathy. But no therapy has shown to work reliably on extreme cases.
Going off what I’ve read, on the more severe end of the NPD spectrum you have malignant narcissism and anti-social personality disorder, which are widely regarded as being untreatable:
https://journals.sagepub.com/doi/abs/10.1177/00030651070550020701
https://link.springer.com/article/10.1007/s12144-019-0147-1
A lot of research strongly suggests that narcissism is actually a personality trait.