Yeah, my wife is an immigrant and the US government makes its best efforts to make immigration as nightmareish as possible. I'll be the first to say the immigration system is broken and needs to be fixed.
Depending on where you live, population collapse is either a myth, meme, overblown but remote possibility, or absolute certainty.
In most western nations, it's not going to be that big of a deal, I wager. In areas with net emigration, especially those that are not good at sharing true demographics with their leaders, it'll be devastating.
I would say population declines within certain tribes leading to changes in political power, and hence distribution of resources, is the central issue in many societies, especially those with very low birth rates.
And the tribes are not necessarily delineated along skin color/region of origin/religion, but even age/education/immigration.
You cannot plug and play 50M young people from Latin America into the US or Africa/Middle East/Eastern Europe into Europe, and expect society to not change in ways that the existing population might not like.
In the US and the Americas it won't have a severe impact. Most countries in America should hold up, and Canada has relatively high immigration levels, so there's that.
Much of Europe and large parts of Asia (China! Japan. Korea.) will face jaw dropping population collapses, happening now and in the near future.
