When it comes to Cluster B personality disorders, DBT therapy is known as an effective treatment for borderline personality disorder. Furthermore, as the borderline ages, their dysregulation and other symptoms tend to reduce, often leading to a somewhat more stable second half of life.
The narcissist, on the other hand, tends to repeat the same idealise-devalue-discard cycle over and over, as they spiral toward worsening symptoms as they age. Most narcissists end life in a schizoid state, having either lost their power to garner narcissistic supply, or having grown cynical as they detach from society and sink into a paranoid life of isolation and emptiness while dreaming of their heyday.