Derek Thompson: Whoever wins the next few elections will face the same arithmetic. The U.S. cannot grow through native-born fertility alone. As immigration collapses, the US population will stagnate and even shrink. Urban economics will buckle. Fields will go unharvested. Homes will go unbuilt. Sick Americans will go untreated. Life-saving medicines will go undiscovered. Many voters hated the era of record immigration. They might hate the era of record deportations even more.