What if SF has so much external demand that it doesn't need to generate any internal demand? As in, who cares whether anyone grows up in the city, because the job market is so strong that every available living space will be taken by someone from out of town? What happens in 30 years when there are almost no kids in the city and almost no adults are ''from'' San Francisco? Does it even matter? Maybe this is just a harbinger of things to come as the birth rate declines more broadly.