The better argument would actually be, I would expect us to have won enough games to get into the play-offs and more if we weren't 23rd in the league at capitalizing off of turnovers.
And your right, our defense wasn't the worst ever or the worst even this year. I've said it multiple times that we weren't even ranked last this year and we were top-half in creating TOs. That is definitely far from the worst ever. We did have very bad individual games, which really skewed the numbers, but we also had games like Philadelphia, when all Dallas needed was a win to get into the play-offs and the defense did it's job. Even though Kelly made it easier by abandoning the run.
And the kicker in that is, we actually lost that game by TOs, yet our offense out-performed the Eagles by over 100 yards passing.