I don't see why the Steelers should change their defensive philosophy just because they are having an uncharacteristic year on defense.
If a team is going to play a basic defense, it has to have top tier talent all around in order to be a decent defense. With all the rules favoring the offense, a basic defense with average talent won't stop an average NFL offense.
Who exactly would you have signed that left after 4 years? How has this hurt the Steelers?