Philadelphia had a population of 2.1 million in 1950. We are at 1.6 million currently with blocks worth of vacant lots. We would need about extra 500k to flood in before we'd be reaching the point you're talking about. Losing 25% of our population has been devastating to the tax base and funding of the city. The revitalization of Fishtown is a major success story and model for other Philadelphia neighborhoods (and a huge increase in tax revenue... all those initial 10 year tax abatements are ending and will continue to mature into their full taxation over the next 5 years).