Birth rates go way down in heavily developed western-style free market systems, but you have to build those, painfully, over time. You can't just have the first world pay to uplift the other 3/4ths of the globe, overnight. Which is what socialism tries do to.
Nor can you just shove that 3/4ths INTO developed first-world nations and NOT have them break down..... which is what globalism tried to do.
Capitalism is the only system that matures into a functioning society where live birth/infant mortality rates are sustainable without having to have 10 kids and there's enough productivity to CARE about things like clean water and preserving the environment. People too concerned with being able to eat dont' much care about cadmium mining runoff, you have to be assured you'll be ALIVE in 30 years to care about environmental issues with 30 year medians on the negative effects coming back at you. But the people themselves at the local level have to build it and learn how to maintain it themselves, they can't just hold out a hand and wish for it or be sold the lie that they're entitled to it.