The two front legs crush and process the suburban houses, and turn them into materials ready for recycling (naturally, being giant robots, the compacted materials are fired off in missiles to the recycling plants). The middle and rear-legs slowly but surely terraform the earth left behind. Flora and fauna are brought to the site via these behemoths, and Mother Nature is restored.
This would appear to be extreme technophilia being used in a Luddite manner, a high tech destruction of abandoned human habitat then restoring it to nature. Will the housing crisis, peak oil, and climate change actually result in a world where the suburbs are emptied out and people return, voluntarily or not, to the cities?
Or maybe, just maybe, we can combine clean, renewable power and transportation with smart energy conservation and avert such a Malthusian world.