There are so many mega cities in the World and it is estimated that over the next few decades over 50-60% of the World's population will live in urban areas.
Will these urban areas support that many humans? We are talking about five to six billion people.
Ah ha, now I've got your attention I see.
Well, what about the virtual reality solution, where people live close together but each of the walls of their personal space is like a flat screen TV and they can project any image they wish, or any amount of infinite space? This VR environment has been proposed, and would be very good for concentrated areas.
It could also be used for living underground in times of let's say a 1,000 years of Earth Surface cataclysmic events that would not support life as we know it.
It might be very good for space colonies, or long-term space travel.
It also lends itself well to allowing humans to live their fulfilled life to its natural end without procreating and increasing population on a planet slowly running out of resources.
A large group or a small group of humans could isolate themselves to start another type of society or civilization based on some other format not currently used.
Many of these types of isolate groups could test the best possible type of society and civilization for humans, which could propel long-term species continuance.
Whereas, a current system of government, society, and civilization might find itself ending of its own device or mistakes, so perhaps you might think about it.