Chung and Meriwether are still great athletes. It's just that BB wants Safeties that can diagnose quickly, set the defense, and also contribute to the pass and run defense. All the surviving Safeties offer that to some degree, with various amounts of athletic ability, and size. Adrian at one time had a surplus of both, but BB is bringing him in to be the coverage LB, and long sought Big Nickle, and to captain and steady, this very young secondary.
James Sanders had the mental aspects, that Belichick seeks for his safeties, it was just that he didn't have quite the size and ahtletic ability to shine in the NFL.