Based on US history, the number seems to be around 15% of the population being foreign born at any given time. This ballpark also seems to be playing out throughout Europe as well. Germany's at 20.9%, Sweden ~20%, UK 19%, and so on. The exceptions that exist like in the Mideast are quite different. For instance Saudi Arabia has around 40% of their population foreign born, but those are all just temporary workers who will never be granted citizenship, and live under a very two-tier system. The temporary residents do not have anywhere near the social impact people who might eventually become citizens, or live in the country forever, do.
Now getting back to fertility collapse. A shared fertility rate of 1 will trend towards you losing approximately 2.5% of your population per year, compounding. So after 80 years, a single human lifetime, you'd have lost 87% of your population. If we're speaking of maintaining population levels with immigration, you'd need to have 87% foreign born at that point in time. So what I'm getting at with the 15% is we can argue whether it's 10%, 15%, 25% or whatever else. But whatever the exact number of 'good' immigration may be, it's much less than 87%.
And furthermore, when you start looking at the actual numbers involved here, it is literally impossible. If we say e.g. 60% of the world, by population, is losing 80% of their population of that timeframe, then there literally are not enough people to replace them in the rest of the world. That'd be a population decline of 48% (of the global population) and only 40% of people in the rest of the world. And that's if you're willing to just accept literally anybody, regardless of whether they have any skills, education, or can even speak your language. It's just not realistic at all.
This is flawed because you're not even bothering to count the 2nd generation or generations after. The effect of immigration doesn't go away after the 1st.
> with rising anti-immigrant sentiment emerging for the same reasons, at almost exactly the same percent.
Yes, you've repeated this from another one of your comments. You're still pretending that assimulation doesn't happen.
"Yes, in the 19th century there was a lot of racism and prejudice directed towards the Irish, German, and Italian immigrants. Not surprisingly, they did not destroy US society or culture. Instead, the US absorbed respective parts of their cultures and assimulated all of their populations over decades. now they are an integral part of America"
> fortunately I don't think environmental or other issues are playing a significant role yet, at least not in terms of direct infertility > It's not existential because it is the most absurdly self-correcting problem imaginable
Unfortunately, there is mounting evidence that the more industrialized a country becomes, the more its populuation becomes infertile. Related studies account for socioeconomic changes i.e. even without socioeconomic changes like the sexual revolution, fertility would still trend down. Air pollutants, endocrine-disrupting chemicals, and heavy metals are linked to reduced sperm motility in men and accelerated ovarian aging in women globally, even in places that are more traditional culturally. The only region untouched now is Central Africa.
> But ultimately it's not even a choice. On top of the cultural issues, I think immigration is simply also unrealistic on a long-term scale.
America has relied on it since its inception. It has only reversed course recently. It's been proven to work for hundreds of years now. Canada has also relied on it since the 1970s since their birthrate was no longer sustainable. They also only reversed course recently.