A lot went down hill too because Brock is a much better pass blocker and receiver than any of the other backs. Norton is a good runner but still has a ways to go in terms of blocking and receiving.
We also went from RB1 to relying heavily on RB3 & RB4 after Brock's injury. After seeing Norton finish the season (albeit not completely healthy), the drop-off after Brock/Norton was rather significant.