We've talked about this a few times in pubs etc. And by your own admission, what young person would *want* to stay in New Zealand, a soon as they can they are up and out. And the ones who do so are normally those who with tertiary education, exactly the population required to drive not just economical but often social and cultural development. i.e. YOU.

Sure NZ also benefits from immigration and the selection process involved, but it's not enough. And it's problem is different from many other small states, who are often close enough to major economies to benefit, a few hours drive/train. NZ is too isolated to benefit from that.