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.