> not even be successfully treated (i.e. divorce)

If one of the partners is narcissistic then one of the partners probably is the problem and divorce is the right solution.

You can't really treat narcissism. You can't make a person who doesn't feel empathy and doesn't want to feel empathy, to feel empathy.

I think a therapist represents change, and change can be bad, especially when someone like a narcissist threatens deep emotional harm or "war" for asserting ones boundaries.

I think it's more of the dynamic between comfort and growth. Seeing a therapist is seeking growth, and it requires discomfort. You can't really have growth without discomfort. So people put it off until their "comfortable" environment becomes unbearable.

Denial is an extremely powerful (and often normal) state to be in.