We cherish that which is rare, which is why the accomplishments of O.J. Simpson, Chris Johnson, Terrell Davis, Barry Sanders, Jamal Lewis, Adrian Peterson and Eric Dickerson will live forever. They are in the NFL record book, the seven men to run for at least 2,000 yards in a season. "Buffalo" and "1973" and "2,003" might not be in the first sentence of Simpson's obituary, but they will make the second.
A 2,000-yard rushing season is an accomplishment so rarely achieved that none of those men, three of whom are in the Pro Football Hall of Fame, ever achieved it again.
That is why, despite his best intentions, Johnson will be hard-pressed to repeat 2009, when as a second-year running back for the Tennessee Titans he gained 2,006 yards to join the 2K club. CJ2K2X we probably will never see.