Eventually either the Narcissist or the Empath, or both, create a crisis where where for their own survival, it is imperative either one or both of them, change and grow. Depending on how unhealthy both are, sometimes this means the relationship must become abusive and extremely dysfunctional, before the crisis I refer to occurs.
Often the Empath will do some serious transformation through the relationship, and the Narcissist won’t. An Empath will get to the point of seeing such an extreme manifestation of their lack of self-love and self-worth, and the ways they give their power away and expect someone else to be their source of feeling good, that they will be convinced they need to heal these core wounds, once and for all, as the alternative is remaining stuck in a relationship that is killing them. Then, eventually, with the right support and (internal and external) resources, the Empath gets out. Or, the Narcissist finds that the Empath is no longer providing the Narcissistic supply they once were, due to the Empath healing or perhaps not being capable somehow, and the Narcissist leaves to find other supply.
I find these relationships are truly done though, when the Empath has finally learned to be their own source of love, and has built a solid sense of self that is alright with being alone. As, if the Narcissist comes back around to hoover them in again when other supplies dry up, they will have grown beyond getting hooked by this love-bombing behavior and the promises of a wonderful fantasy future together.