As much as I HATE the Eagles, let's give credit where it's due...
Quite simply, their FO-they not only draft well, but under Reid's/Heckert's philosophy, they utilize the draft to prepare for their (far)future, which is why they rarely go out on a spending spree, in terms of FAs, that is.
After their last SB, that team, especially their D, just got old. But nonetheless continued to use each draft to prepare for their (far)future.
So ultimately-when one window closed, they were able to open up another. Cowher was able to do the same thing while in Pitt.
And, of course, they have the luxury of not only being able to hold onto Jim Johnson(he's too old to get a HC gig) and have Brian Dawkins. Without either, they would be mediocre.