Age alone means they should be looking to replace him.
I've looked for salary numbers for the top starting CBs on each team in the league. I can't find good numbers, but I'd guess that $8M puts him someplace in the middle or lower half of the list.
As to his skills, he gets beat on double-moves. And he's tricked now and again. That's not an age thing. The double moves didn't burn him as badly previously when we were getting to the QB faster. He's still plenty fast, which you can see on his recovery speed when he does make mistakes. There's no shame in running stride for stride with Desean Jackson after you recover from biting hard on a route.
The guy's been underrated his whole career, and even though he's at an age where he's obviously going to start declining, it hasn't happened yet. I think we find someone to take over for him next season in this draft, if we're lucky. Otherwise it's an expensive FA acquisition next offseason.