President Barack Obama says he has enrolled 7.1 million Americans through his new health insurance exchanges. If so, that may prove to be the easy part: the Affordable Care Act creates so many incentives for enrollees to drop their coverage that maintaining those enrollment numbers may start to resemble something like pushing millions of people up a greased poll. For Obamacare to work, people must enroll and stay enrolled. An estimated 20 percent of those who signed up have yet to pay their first premium, and as many as 5 percent stopped paying after the first month. If too...