The housing prices are extraordinary and the cost for people to live so far north is significant. Maybe we don't need a lot of people being born up north. There's certainly enough people on the planet to make up for any headcount-shortfall with immigration (and in Norway in particular, quite a few people born there move away these days because cost of living is so high).
People in more developed countries are >10x more productive than people in undeveloped countries.
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. ;)