While I agree that a lot of progress in commerce, technology, and many other facets of civillization were made possible by cities, there can certainly be a point where cities are too large for their own good.

Cities--especially megacities--are such complicated creatures, it's hard to really make blanket statements. And they are different from each other so what works in one place may not work in another. New York City and Tokyo are vastly different on many levels from Lagos and Rio de Janeiro.

However, as the human population continues to climb, more and more cities will enter the ranks of megacities, so it's certainly an important issue.