Well, that is under the assumption that he would experience an immediate decline right after going off steroids. You quit steroids and you don't lose everything you gained overnight. It can take a long while.
Also, 7.5 of his 12.5 sacks in 2007 came against the Raiders and Chiefs (the combinations of Daunte Culpepper, JaMarcus Russell, and Damon Huard). Did he get 12.5 sacks because he wasn't declining or because he faced two of the worst offenses in the league. In 2006, he had three sackless games. In 2007, he had seven sackless games.
Lastly, Merriman's knee injuries could be a product of the roids. Many different types of steriods build up muscle, but weakens the ligaments that attach them. Many steriod abusers suffer torn ligament injuries because of it.