E.g. if Canada only searches out the best and brightest from struggling countries, that delays the success of those countries be decreasing the average capability level of improving conditions. This of course works in Canada’s favor, because it needs poor countries with conditions that facilitate child rearing.
A lot of technology is making things easier in struggling countries, and if that technology can be made cheaper and commoditized it can improve more lives at more points of the income spectrum. The best and brightest may be helping to eliminate their own positions by making certain jobs easier to do in more places on the planet.
On the other hand if the output of their productivity isn't flowing back to their home country, Canada/USA/UK/wherever are reaping the rewards tax/tariff-free and the increase in productivity isn't being redistributed.
You wouldn't let a foreign power extract oil or gold for "free" from your country, would you? I don't want to argue against personal liberties but at the same time it could be argued that those rights are being exploited by rich countries to the detriment of poorer countries.
Then there's the whole TFW (temporary foreign worker) program that looks a whole lot like serfdom where landowners in Canada buy labour at below market costs without even extending the notion of citizenship to the workers...
Yeah, I'm not from Canada so you obviously know a lot more about this than me, but essentially 100% of my Canadian friends and family cite this as their top or near top concern right now. Toronto in particular seems wild, though Vancouver may be worse.
Moving to Canada improves their standard of living.
Global Sentence: Global Population = People born.
Once every country is pro immigration without being pro birth there will not be enough births to maintain the current population. The can is being kicked down the road.
The problem isn't the shrinking population though. It's the aging of the entire world. Someone has to take care of these people or they die alone with nobody to attend their funeral.
The truth is letting people die by the millions in a war tends to take care of problems like this.