That estimate is no longer current. The latest estimates has the world population going well past 10 billion, driven by growth in Africa: http://www.un.org/en/development/desa/news/population/2015-r...

That doesn't mean that people won't concentrate in very dense cities.

What's your basis for such a strong claim?

https://esa.un.org/unpd/wpp/Graphs/DemographicProfiles/ has data if you want to make a statistical argument.

Source?