Why do we care about Earth? Earth can handle its own affairs, and it's not overpopulated, there's still hundreds of thousands of square miles uninhabited.
So.
Sandy Africa can support people?
China. You do know that 3/4 of it's land can't support crops.
You have to look at the big picture.
You need fertile land to grow crops.
You need fresh clean water for the crops, animals, and humans.
You need fuel for heat, cooking, transportation.
You need a place to put your sewage.
Only a very small portion of Earth is suitable for humans.
Take Alaska for example. It is the largest state in the USA.
But it has the smallest population.
Why?
Because if you really want to live there. You need to live like a Eskimo.
Most people leave the state because it costs to much to eat and heat there homes.
You could kill 2 mooses every year, to help feed your family.
But most people don't have the stomach to skin and dress some tasty meat.
For heat they could chop down some trees for firewood.
But some people are too lazy for manual labor.
I could go on.
But it's pointless to try to explain the reality of over population to any one who does not live in China, India, or Tokyo (Density 5,847humans/km2 15,143.7humans/sq mi).