For thousands of years (tens of thousands, probably) cities were a demographic drain. More people died in London than were born there, and the cities only survived because you can make a lot more money there (including enough not to starve to death).

Perhaps in the future the same thing will happen: kids will be reared in cheap places, then move to cities for the opportunities there, then fail to produce at replacement levels, and get buffed up by people moving in to the city from the country.