Last time I checked, annual births were at 130M and deaths at 60M giving a net increase each year of about 70M.
Forget the 1-child policy. Even if you had a 0-child policy, population would only drop at 60M per year as people died. That's assuming we stop any further development of medicines etc. etc. etc.
So, to get from 6.5B to 3B would take 60 years. Of course you can't have 60-years with no births because then the youngest woman would be 60 and unable to have children anyway. To get to 2-3B population in a managed way that produced a stable population at that level that didn't oscillate wildly would take hundreds of years and involve the cooperation of everyone on the planet. That's too slow.
In short, forget it. It's not going to happen. We are going to have to find a way to feed 10B+ people or go through the kind of extinction that hasn't been seen since the dinosaurs went.
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