French investors will breathe a sigh of relief this month, as the country’s supertax on high incomes quietly expires. The supermistake won’t be missed.

François Hollande promoted the 75% tax rate on earnings over €1 million ($1.2 million) to fire up his Socialist base during his 2012 presidential run. Once enacted, it proved an immediate fiasco. High-earning soccer players threatened to go on strike over the rate. LVMH boss Bernard Arnault, the richest man in France, threatened to obtain Belgian citizenship as the tax went...