the only sensible 'world population' is one that can achieve "Steady State" equilibrium...that is, where we can eat all the seafood we wish and chop down all the trees we need,and generate all the 'safe' energy we need without creating shortages...(and there is land available to REMAIN farms and not be turned into highways & strip malls)...

we passed that size population 40-60 years ago. Conservation and plant research and clever uses of energy can help, but the fact remains that we breed faster than resources can be developed. The core issue is called 'carrying capacity'.

If you want a clear example, look at the old Indian ruins in Mesa Verde Nationl Park....they had ladders they had to climb to get anywhere, a limited supply of water and only so many places they could build cliff dwellings...and there is overwhelming evidence that Nature gave them a climate change that made their setup impossible to maintain.