First, we were talking about whether it was a good move, not whether the team subsequently did as well as hoped. Those are different things.
Second, the way you presented it is an oversimplification because that move alone doesn’t determine the success of the team. There are 11 positions on defense and between starters and role players maybe 16-18 players manning those positions, and all play a part, plus there are all sorts of other factors like level of the opponent and injuries, and game plans, and how well the offense does at keeping possession of the ball to limit the opponents offense, and schemes and any number of things that all play into how the defense as a whole performs.
If some of those variables cause problems it doesn’t mean an individual deal was bad.
Teams don’t get the hindsight benefit of knowing If the entire team or entire defensive unit is going to holds their end of the bargain when they make a deal. They can, however, improve the talent level and add tools. They did that with Bennett.