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I don't think 500 million should be the population of the world just because a stone somebody made 40 years ago said so.
There are pros and cons to all population sizes. I would lean towards fewer however but that has it's own issues as we are going to see in the future with many countries having ageing populations and not enough young people.
But keeping the population at the size it is is unfeasible for two reasons, firstly we don't have the resources and secondly it won't be long before the developing world starts reducing in population as well.
So, we have to find some way to manage in an ageing world. Whether through technology, societal changes, austerity or all three.