A large part of today's population already resides in big cities around the world. Would it be more plausible to research on how we can improve the situation in the current cities, instead of starting from scratch.

It's like Software, where you come up with a large code base that is ridden with problems, but somehow works most of the times. You can either dive into that and improve it or you start writing your own system from scratch.

I am sure YC would have thought about this. Just want to know why they choose starting from scratch.