>Environmentally, it's a fantastic thing considering the frightening rate of deforestation, species going extinct, carbon emissions, and ocean pollution.

This assumes that the amount of resources consumed per person is relatively constant. That's not a good assumption. Fewer people can easily hog more resources, especially energy. People spreading out and buying more and bigger cars and living in bigger homes means more energy and pollution, whereas if more people concentrate into a dense metropolis with public transit and tiny apartments, their energy needs are lower per capita.