You're absolutely correct. Do you understand how to reduce those fertility rates in shithole countries? By developing those countries.
Do you understand how to develop these countries? It starts with not depleting them of their non-violent, hardworking, and/or skilled citizens and leaving them with their shit citizens.
It's not "by default" that the same will magically happen to undeveloped countries.
But also at the same time - once countries like India DO manage to develop (as one of the few that actually is developing) - guess what? Time to haul in shit tons of energy using things like air conditioning. Naturally when a country develops their KwH will explode. So your plan there isn't exactly smart.
The first part is poverty reduction which objectively reduces birth rates better than any other viable solution. (See Japan, Europe, and the US minus immigration).
In fact current projections show the rate of population increase is slowing. Absolute population should peak this century. We should hurry that peak along.
You are absolutely right that countries that see poverty reduction will use more energy. But that energy doesn’t have to be fossil fuels. Wind and solar are already competitive. The stupid part would be using coal and oil and making climate change worse.
If we were helping guide the world this direction there would be plenty of business opportunities in helping the third world quality of living improve.
Market sizes and GDPs wouldn’t even have to shrink as population shrinks after the peak thanks to increases in productivity per per worker.
Doesn’t this sound like a better way to a smaller less harmful population than hoping half the population dies?