Your original post was saying that it is OK for population decline in the northern countries because it is too expensive to live there. And population growth in other (less productive) countries is fine because it evens out.
1) There's no set "right number" of people who should be living in northern countries, no rule that says we need to replace every emigrant (or unborn person) with an immigrant. If it's too expensive to live up north, maybe just... Fewer people just live up north.
The population of Norway, right now, is higher than it's ever been in the country's history. Maybe a population decline looks more like a correction to a "right" amount?
2) To the extent that more people are needed, one can assume that the headcount shortfall from lower birth rate will be filled with skilled immigrants who like the climate and bring a competency that creates enough value to afford the cost of living.
NOBODY likes the climate.
There's no bad weather, just bad clothing. ;)