The experiment looked doomed at the start. Five years ago, General Motors, Ford Motor, and Chrysler, with the blessing of the United Automobile Workers union, created an independent entity to provide health insurance for more than 700,000 retirees and to manage the investments that would finance the benefits. The move was good for the automakers, allowing them to shed crushing liabilities that threatened to impede their recovery amid the Great Recession. The conventional wisdom was that the trust fund would quickly run low on money and ask union retirees to cough up large annual premiums or settle for more limited coverage. “People didn’t believe the math worked,” recalls Art Schwartz, GM’s labor negotiator at the time. Contrary to expectations, the $61 billion UAW Retiree Medical Benefits Trust is thriving. Retirees’ drug costs are falling; dental, vision, and other benefits have even been added. What’s more, the investment fund that pays for all of it has booked double-digit annual returns in recent years…..