Cornerback is one of the loneliest positions in the NFL: it's often just you and the WR on an island. You can have all the knowledge and technique in the world, but if you don't have the speed to run and cover the guy across from you, your a liability.
That's why the average age of a starting NFL CB is only 26 years old. Last season, Terence Newman was 33.
Under Jerry Jones, the Cowboys front office has traditionally been unwilling to trust their scouting department, squeezing every last bit out of veteran players before letting them go.
As a result, Dallas is always a year too late rather than a year too early in moving on. At a position like CB, that kind of thinking can cost you a season.