They play different positions. For what Williams does for us YES and it's not close. Mathieu is a move safety (plays both FS, SS, and CB.) But he's not as good as Marcus Williams as far as a pure FS.
Williams is in a deep shell I'd be willing to bet on 95 percent of our plays, whether that's cover 1, cover 2, 2 man under, cover 3, cover 4, or cover 6.
And no one in the league is better at erasing the deep ball than he is. No one on the list you posted earlier and certainly not Mathieu.