#1 - You are assuming the Offense will remain at its high level while we ignore it in order to commit the high level resources needed to fix the defense. If we "fix" the defense only to have the offense fall apart, we'll still be in the same position. With the likely departure of Williams, Witten a year older, and Dez seemingly being a lot more injury prone nowadays, it is not far fetched to see our receiving game go downhill if we don't add quality resources there.
#2 - You are assuming we "hit" on those defensive resources we bring in. We have proven to be great at picking offense with premium picks but we have not proven so with defense. Claiborne and Jones, while both okay, are not home runs which elevate our team as first round picks should. And free agents, no matter how productive on former teams, are usually a crap-shoot, often being busts with there next team.
#3 - Even if we do as you said and our offense stays great and our defense improves greatly, it still doesn't guarantee us a Superbowl. Injuries, inconsistent coaching, bad refereeing and just bad luck can derail any team.
So no, not simple as that at all.
That being said, I agree we need to lean toward defense in all our immediate decisions as it is still our most glaring deficiency. I just disagree with your "simple" and seemingly guaranteed SB portions of your statement.