There are not any advantages in a population of 20B over 8B. Just more world hunger. The world is so wedded to the profits of capitalism that any alternative is dismissed out of hand, yet capitalism demands growth and over consumption. Attrition through starvation and disaster are acceptable but planning to reduce the population is unthinkable. What's "politically unfeasible" today, will look like common sense tomorrow. But by then it will be too late. It could be that we are too selfish and self centered to own up to our self-destruction.