He's gone from a 2nd pairing guy who struggled defensively but who put up a ton of 2nd assists on the Sedins-Ehrhoff PP to a legit top-pairing shutdown defender.
Yeah. I wouldn't say he's really "declined" at this point, so much as he's just...morphed into something a bit different than expected.
The offensive side of his game never really went anywhere and sort of dried up when the mirage of that high flying team no longer padded his stats to the same degree. The physicality has waned as well. But he's become a much steadier presence overall.
At this point, he's still a quality Top-3 Defenceman. A guy who is either a solid complement to a 1st pairing, or a guy who could
anchor a very good 2nd pairing on a deeper team. Even the offensive results would likely come back somewhat if he were to find himself on a more talented and deeper team again. But right now, he's being asked to masquerade as true #1D, which he's not...and never has been.