Folks have been importing top soil for a long time too. That doesn't mean that you get to be okay with soil erosion. Or for a better example, folks have been digging wells and building canals for agriculture for as long as civilization has existed. That doesn't mean you get to deplete your aquifers and natural reservoirs.
I want sustainable to be sustainable. Responsible societies should be able to maintain and improve the people that they have. Any people imported should be a supplementary augmentation, calculated for sustainable growth and improvement of the society as a whole. An augmentation, a supplement, is very different from a replacement. A replacement is what happens when you have the irresponsible depletion of human capital, and frankly, any immigrant should not want to go to a place where human capital is unsustainably depleted in such a way, as their own offspring are likely to receive the same treatment.
It is akin to assuming that you can import soil after you have depleted your own. Human ecology matters as much as soil ecology does.