Tim Cook is ending his illustrious stint as chief executive of Apple. The soft-spoken operator accomplished the near impossible, filling the shoes of the visionary co-founder Steve Jobs, turning the iPhone from a cultural phenomenon into a financial juggernaut and transforming his company into a $4 trillion goliath — growing its market value by $682 million per day, on average, for 15 years. By the metrics investors care about, Mr. Cook is nothing short of a rock star. But when one considers his role in the sweep of American history, his legacy grows more complicated, for much of Apple’s success...