Climate Change vs Population Growth one of your comments addressed the issues I raised;
1, will sub-Saharan Africa be able to cope with more than doubling its population within 35 years and
2, What would be the effect on its environment and greenhouse emissions.
Immigration is definitely not a solution to a problem of this scale even if it was not fiercely resisted by the hosting countries, It is worth noting that infant mortality nowadays even on the poorest African countries is less than 10%, so fertility of 5-6 children is not justified on grounds of being looked after in old age.
I wish I could be as optimistic as you are.
Incidentally, China would not be where it is now were it not for the one-child policy/
Also, please bear in mind that automation reduces the need for manual labour. A Mac restaurant today employs less than half the number of one in the 80s. A car factory of say 4000 workers produces more cars than one which employed 20,000 in the 70s. Robotics makes major inroads been into the caring services,