The “moderates” in the Democratic presidential primary hope to build on the Affordable Care Act, rather than make the more dramatic shift to Medicare-for-all — and even Sen. Elizabeth Warren (D-Mass.) has changed course to embrace a plan that would move to Medicare-for-all more slowly. But if they want to build on the ACA, they need to confront a serious perversity of the private insurance system underlying it: The system’s financial health depends on people choosing the wrong plans.