[rtkwe]

To change an existing city you have a lot of baggage in the form of existing structures and landowners who you have to either convince that the change is beneficial to them or force the changes and fight the inevitable legal battles. That's just assuming that your plan can be implemented with the existing structures, if not it's going to require purchasing the existing structures and rebuilding/tearing them down.

That's not to stay starting over is easy. Trying to create a city out of whole cloth with sufficient jobs and infrastructure to attract and retain people isn't an easy problem in the slightest. It does avoid the NIMBY fight of trying to change an existing city though.