Growth in the center has now reached the point where the challenge isn't saving inner-cities, it's working to maintain affordable housing. Research has shown that limited access to employment opportunities is a huge burden for poor households. In the 1980s and 1990s, lower income workers often lived in central areas while jobs were increasingly located in wealthy suburbs, unreachable except by car (which many poor households are unable to afford). As job growth in central cities takes off, it's important to ensure that lower income households maintain access to employment opportunities.
Philadelphia's somewhat strange is that even as the center has boomed there are still plenty of not awful places which are still somewhat affordable. Obviously for the very poor affordability is always an issue, but for those of modest but not awesome means Philadelphia is very affordable relative to Boston, Chicago, etc. But basically the city really needs more mass transit, and to some extent better exploitation of the existing transit system.