They leave when they have literally sucked the life out of a person. This person no longer admires them, but stays because he or she is trauma bonded. Once the abuse has taken a toll this person can no longer supply the narcissist. The narcissist needs a steady dose of supply to keep the false construct in place. If you have been depleted due to abuse you no longer respond to the devaluation, the gaslighting, the constant criticism, the scapegoating, and the circular arguments which make no sense. Your own feelings have shut down in order to protect yourself from the abuse. Your responses or non responses to all of the above are no longer supplying the narcissist. He/she views this as a failure on your part. You have failed him and you must go. In fact the sight of you sickens him. He’s also been busy looking for your replacement. People are interchangeable as sources of supply. He may try to circle back around once you have recovered.
If you have been discarded take this time to heal. Once the fog begins to lift you will see the truth. You were toxically tied to an illusion. A person who has to abuse others in order to exist. The truth is the narcissist is fueled by your emotions during Devaluation. It gets worse and worse because it takes more and more to fuel him. You become stale to him when you become numb to his treatment of you. They will literally suck the very last drop of fuel out of you at discard. In the end you can never forget the cruelty of this person, as the narcissist unmasked is an ugly sight to see. There is nothing human there.
During the relationship you may have seen glimpses of the mask slipping as he vacillates back and forth between Jekyll and Hyde. He will always have some excuse for his abusive behavior. At the very end of the formal relationship the mask is completely off and all the rage, anger, and cruelty comes pouring out. He is angry and disgusted by you and your weakness. You have failed him, you are broken. If your television stops working you would have no problem replacing it. And you would have no problem throwing the old television in the dumpster. This is what you mean to the narcissist he has no bond or no tie to you. I don’t care how many years you spent with him. He does not have the ability to bond as a normal person. He does not care about your feelings. His empathy never developed or was muted in childhood. He is not capable of caring for another. There is only room for one. The narcissist can only care about himself. Because of his maladaptive defense mechanisms he is too self absorbed to care about another human being. The entire life of the narcissist is about fuel/supply in order to keep the false construct in place. A narcissist will leave a relationship when his partner/spouse is no longer able to supply him.