I don't like the term decoy in football. If a DB and a LB jump in the flat to cover a fast RB, and that leaves a TE open over the middle (exact same thing that happened w/ Sproles and Graham in the Tampa game) to me that is a well executed play design, not using the RB as a decoy. If the LB doesn't jump then the RB would have been 1 on 1 which is ideal in open field.
The 1 and only reason the offense didn't suffer was because we signed the only FA who could fill the speed void we had. Statistically your claim may be correct, but let's not act like it wasn't a big hole to fill. If Sproles would have signed to another team our offense would have had a pretty big problem.