Sometimes we wonder if Pete has lost his touch? He hasn't produced too many defense stars in the past few years, before that anything he touched turned gold.
Talent and personnel certainly is a part of this equation, but so is the rest of the league figuring out how to crack Pete's cover 3 defense.
That's the NFL, whatever new flavor there is on both sides of the ball (Tampa 2, west coast, run and shoot, Air Coryell, No huddle Greatest Show on Turf, etc.) All variations of other offenses and defenses that came before through their coaching trees.
Eventually the smart guys on the other side of the ball figure out how to counteract it.
So IMO if we're parsing as to why Pete's defensive effectiveness has diminished, #1 is probably talent, but 1A or 2 is his stubbornness to change his defensive and offensive philosophies. It seems like he finally is with the hiring of Waldron and changing up the personnel on his defense to smaller quicker twitch CB's, LB's and a more versatile D-line.
We'll see if it was too little too late.