If Detroit were its own city-state — a bizarro version of prosperous Singapore — it would be a place the U.S. State Department would regularly issue travel warnings about, where “long-term, protracted conditions make a country dangerous or unstable.” Detroit’s murder rate, for instance, would be higher than that of all but a handful of other countries. The Democratic Republic of Detroit would also be relatively poor, with annual per capita income of around $15,000, making it a middle-income country like Mexico. Except Detroit would be a nation in decline rather than on the rise. Detroit would be a nation...