How megacities are changing the map of the world

I used to saw mega cities as monstruosities of civilization. A Frankestein monster that should not be made. Until I reach this paper: http://www.pnas.org/content/104/17/7301.full.pdf
So, we may dislike or not big cities, but they exist for an economy of scale not easily grasped to the naive observer. Ironically, the droves of people gathering in megacities seem to understand the sensibilty of doing that, even if they cannot explain clearly the reasons.
In the past some resources were the key for wealth and power: land, capital, an army, etc. I believe the wealth now goes to the herd of “prosumers”, it is producers and consumers. The rational is: Raw materials are sustituted and obtained in alternative ways, and so none of them is a key asset for long time.
Manufacturing facilities, and I am including here not only factories but also communities of artisans like the egiptian workers of the pyramids, the glass makers in Murano island, etc, are not longer absolutley required, for the knowledge and tools are spread all over.
What stays is face to face interaction: people is the source of needs and its solutions. People consume the services other people bring, and the differences between them is slim. Most of us fill both roles.
For this a city that eases the interchange, the transportation of things and ideas is a winner in terms of eficacy.