Rich people across the Western world are anxiously watching France, where president Francois Hollande is vowing to raise the top tax rate—on earnings over $1.2 million a year—to 75 percent. Tres bien, Mr. Hollande. The problem with this otherwise fine idea is that the very rich can simply pack up and move to a more accommodating Western nation with lower taxes and less concern for income inequality, like America. There is, though, a more elegant solution to this: a maximum income.