Sorry for not answering earlier. This thread is moving fast.
My answer is: I don't know. All I can base my opinion on is the fact that they've been making good adjustments on both sides of the ball, all year long. I am not going to pretend I have the knowledge to go into detail about their defensive adjustments to stop a running game. I can only tell you from the perspective of a fan and a spectator.
With that said, someone else (maybe it was you) made a point about removing the threat of the read option and that allowing the Seahawks defense to simplify their strategy and focus purely on Morris as the sole running threat. I don't know. But, I do know that Morris was gashing our defense in the 1st quarter even on conventional rushing plays, and that slowed down a lot after being down 14-0.
I'm also not completely sold on the notion that Morris's NFC rushing title was the result of RG3. The dude's a beast regardless.