How about until we've turned them into Boston? Boston and Manhattan are congested, admittedly, but Boston at least (I can't personally speak for Manhattan) is a wonderful place to live. It's a normal, natural, _human_ city, with medium-height buildings and mixed commercial and residential land use (businesses at street level, residences above and sometimes below); it's a far cry from the typical suburban wasteland, half Le Corbousier and half Leave it to Beaver.