Vic Fangio had "pieces" in his four years with the 49ers. And, man, did he put them to use. Units that ranked fourth, third, fifth, and fifth overall in the NFL made San Francisco a contender, no matter where the 49ers were offensively.
Alas, Fangio was bypassed for defensive line coach Jim Tomsula to replace Jim Harbaugh, heading into 2015, freeing him up for John Fox and the Bears (whom Fangio chose after an interview in Washington, as well). Following him out the door would be most of his bread-and-butter players: Patrick Willis, Justin Smith and Chris Borland (retirement), Aldon Smith and Ray McDonald (drug and domestic violence issues), not to mention a 2014 campaign without NaVorro Bowman and his blown-up knee.