If your city grows faster than its historic average it ceases to be able to meet its affordable housing needs through market-based filtering. High-income newcomers outcompete anyone lower on the income scale for rents, forcing everyone else down a rung on the ladder, and the bottom rung falls into homelessness. Then the stock of low-income housing decreases, as the competition for homes on the higher-income rungs makes it worthwhile to fix-up depreciated homes in good locations, and then in not-so-good locations.