Only one little problem... unless you plan for all those nifty new plants and grass to die after planting, you're not going to let the soil dry out.  You're going to irrigate it to keep it moist.  

Which, BTW, gets back to the significant cost element for having to build roofs to support the weight of all that heavy, moist soil.  Plus planter boxes, plus trees/plants.  Plus of course the extra cost of materials and weekly landscaping services.  

Although that does raise an interesting question... is it breaking the law if you build all this but then just let the plants all die because it's too expensive to keep maintaining it?