What's wrong with smaller cities, with more evenly distributed population across countries, with density lowering as you move from the center? Reduces city impact, still promotes dense urbanization and reduces transportation costs if there is something being produces around the city. Also left to nature is a bit impossible in today's world and promotes detachment from feeling like part of nature for most people. Most of Europe and north America is riddled with invasive species or just have been devastated by agriculture or deforestation in the last centuries and if managed correctly which can mean leave it alone humans can have a positive impact on the world. If you could create forests in the desert with plants from around it, wouldn't that be positive?