He could and probably will, have value for many years.
CB is now probably the single most important position on defense in the NFL.
It's also a position where many of the great ones aged very well. You can go back to Darrell Green who played 20 years at the position and was still probowl-caliber late in his career. When he was more than 10 years older than Gilmore is now.
Champ Bailey. Sherman right now is still a very good cb. There are many examples.
You don't trade building blocks. If you have a young,talented QB,cb or ol you don't trade them. They're all positions that age well and also happen to be the 3 most important positions in the NFL game now.
Rb? De?lb? Wr? By all means, those canbe trade chips when they're still young. They have limited shelf life and are much more replaceable.