BIG LAW enjoys a long life. For an illustration, examine “The Partners: Inside America’s Most Powerful Law Firms”, by James Stewart, published four decades ago. With a few exceptions, the book’s starring firms remain in excellent health. Cravath, Swaine & Moore, Sullivan & Cromwell and Kirkland & Ellis are still some of Wall Street’s busiest limousine-chasers. Less fortunate firms have fizzled out, rather than blown up. One shop,…