Cairo, Ill., has a population of fewer than 3,000 people. A plan to demolish two public housing developments would force many residents to leave and cut the school enrollment in half. ___ The authorities announced last month that the cost of fixing the developments was out of reach, and that replacing them altogether would cost almost 10 times as much. For hundreds of residents, the decision may mean not only leaving these crumbling buildings, but also moving from Cairo altogether. “For sure they needed to fix this place up a long time ago,” said Nena Ellis, 38, a mother of...