The battle over President Park Geun-hye’s proposals to liberalize South Korea’s labor laws is intensifying, with subway and railway workers jointly striking last month for the first time in nearly two decades. Since the current legislature is determined to block Ms. Park’s labor bills anyway, the militant resistance may seem odd.

But the unions are right to be nervous. Their influence has waned as public awareness of the need for labor reform has grown. When Korea elects its next President in a little more than a year, the...