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Other cities and states - most notably Portland - have already done this. Maricopa and/or Arizona would basically draw a line around the Valley beyond which development isn't allowed.

Another way to do it is to create a ring of wildlife preserves.
And yet another way to do it is informally, by refusing to publicly fund the extension of utilities beyond a certain point.

This protects the economic, financial, and environmental interests of the region by cutting off further sprawl. The urban areas get more focus on density, and the ex-urban areas don't lose their unique location on the edge of the Sonoran desert.