Some countries and economists are worried that falling birthrates will create economic problems. Not enough young people to work, shop and pay for the retired folks. Changing demographic distribution.
On the other hand, countries, like USA, Canada, much of Europe and Australia have over whelming numbers of young workers wanting to immigrate here. Seems like so many people, the world over, have the dream of coming to America.
Next problem is, how do we find housing for all those people and if they all drive cars, where are they going to park?