For several years, Laurence D. Fink, chairman and chief executive of BlackRock, the money management giant, has been on a crusade, exhorting corporations to change their short-term ways. Executives should forgo tricks that reward short-term stock traders, he argues, like share buybacks purchased at high valuations. Instead, corporate managers should focus on creating value for long-term shareholders. It’s an admirable argument that has won Mr. Fink wise-man status on Wall Street and accolades in the press. Hillary Clinton has echoed his ideas on the campaign trail.