I think you have several things quite wrong. We have eight or nine Safeties on the roster and can only keep five at most. Geno, when he was healthy, was a near pro bowl FS. When Rodney went down in 2005 there was nobody except an immature rookie to play SS, and the Pats went through a succession of one week failures playing alongside Geno. When Hawkins showed up, Geno got hurt.
IMHO the Pats have a superb secondary composed of a Pro bowler with Asante, a very good young corner in Hobbs, and proven starters in Gay and Scott for depth at CB. Their Safeties are Wilson and a now-experienced Sanders, with Rodney and Hawkins and Gay for depth, and Baker, Andrews, Scott, and Mitchell for even more depth.
With regard to Lance Briggs, he has said he doesn't fit in a 3-4 and he ought to know. He complains that he plays second fiddle to Urlacher at LB in Chicago. How would he like to play second, third, fourth, or maybe fifth fiddle to Bruschi, Vrabel, AD, and/or Colvin? That's why it would never happen !