Taking a long term view, half a century or a century out, at the rate the world is currently developing, it seems possible there will be no poor countries left to supply cheap labour. Most of the world will be wealthy, except for a few basketcases mired in warlordism and economic isolation, such as Congo, Burma, or North Korea today. Such regions will certainly have still have plenty of poor desperate people - but they will not have the education, etc., that the migrating masses driving the economies of the highly developed countries do today. They will be malnourished and illiterate and no one will want them as immigrants, just as they are unwanted as immigrants today. The rest of us seem to be on track to be part of one competitive globalized labour pool.

Because birth rates are dropping in every single country as people get educated and etc. 2nd generation immigrants are not having as many children either after the migrate so it is simply a stopgap until every country has a birthrate less than 2.1 (or whatever is needed to have stable population). That’s why I got curious what is this magical way of fixing the population decline.

Because the assumption that the places immigrants come from will continue to have high birth rates forever is unfounded.