A residue of it survives well into adulthood. It is essential, it keeps us alive. It drives us to achieve things and to seek the approval of other humans. It helps us bond with significant others, motivates us to raise children, to consume, to study, to explore, to discover, to invent, to innovate. It is a powerful engine of human personal progress.
Pathological narcissism has very little to do with healthy narcissism.
It is a malignant form of narcissism because it takes over the host and then kills it. You make NPD sound like a kind of parasite, both in the way the disorder impacts the narcissist himself, and in the parasitic attitude the narcissist then takes towards others. Pathological narcissism is parasitism. It is the unabashed, ruthless and unscrupulous exploitation of others as sounding boards, as accumulators of past glories, as servants, as extensions of the narcissist. The narcissist idealizes, then uses, then devalues, then discards.