An aging world requiring fresh human stock. Where will these people come from?

The US, Europe, half of Asia, all are demographically old nations with the western world being kept aloft through constant immigration from the rest of the world.

Currently 48%of the world lives in nations with a below replacement fertility rate and this number will only continue to rise in the years to come.

So the question that follows is, where will the world get the required manpower from? How will they get the necessary skilled labor supply?

North America uses Mexico for it's supply of human labor. A resource abundant enough that it can last this entire century. Canada too manages to accept enough people from all over the world to continue to expand year on year.

However Mexico itself has reached a fertility rate of 2.1 and based on previous data will see population decline within 30-50 years. Mexico itself may fill it's own population with migrants from the south American nations, but the south American nations themselves in turn will by then be seeing population loss.

Europe would get extra bodies anytime from North Africa and the middle east. The middle east in turn has Saudi Arabia and the UAE with enough wealth to constantly bring in more people every year, with many temporary migrants coming in from Asia, yet even for them the same issue arises, the Asian nations too are in part already seeing population decline, and will themselves have to look for other sources to get more labor from.

Africa becomes the new manpower breadbasket of the world in this scenario. The only continent in the world abundant in fertility and with many children in the coming generations, but does it have the capacity to build enough skilled manpower to supply the rest of the world? Along with the necessary population?

The population of Africa by 2070 is expected to reach 3.3 billion. The global population is estimated to be 9.4 billion. Thats 6.1 billion people all in countries that need additional labor from Africa to keep things running. An Africa which will itself likely start to slow down in growth around that time period.

Assuming the rest of the world takes in 10% of Africa's population to keep running smoothly,(10%of Mexicans live/work in the US currently and are the primary source of cheap migrant labor for the US) that would mean that Africa needs to provide 944 million bodies to the rest of the world by 2070, around 25-33% of the total population of the continent. Which is on the face of it an impossible demand.

So there won't be enough people around for all societies to keep growing even if they want to keep taking people in. These societies will likely not collapse but some of the nations of the world will almost certainly wither away unless they are accepted by a neighboring territory under a larger nation banner.

So my argument is thus, two things in our future are set, there will be societies that will be stagnating and dying out no matter what we do, and there will be new nation states born as an amalgamation of shrinking nation states in an attempt to remain relevant for a few more decades.

Federations such as the US, Europe, and India aren't an ideology, they are an inevitability, and when your neighbor becomes too huge to fight on your own, wouldn't you too like to be part of a union to balance things out?