For those who appreciate verisimilitude in their cable TV dramas, a highlight of this week’s “Billions” season finale on Showtime came when a young prosecutor told her boss, a fictional Manhattan U.S. attorney, that she was leaving for a real-life law firm. The show smartly name-dropped Sullivan & Cromwell—the place to be for upwardly mobile securities fraud experts looking to trade a government paycheck for something much, much more lucrative.