Defense is a fundamentally harder problem than offense.
What matters is not whether defense is “harder” than offense, but what AI is most effective at improving. One of the things AIs are expected to be good at is monitoring those “360 * 90 degrees” for early signs of impending attacks, and thus enabling appropriate responses. You can view this as an “offensive” solution since it might very well require some sort of “second strike” reaction in order to neuter the attack, but most people would nonetheless regard such a response as part of “defense”. And “a huge surplus of distributed offensive power” is of little or no consequence if the equilibrium is such that the “offensive” power can be easily countered.
What matters is not whether defense is “harder” than offense, but what AI is most effective at improving. One of the things AIs are expected to be good at is monitoring those “360 * 90 degrees” for early signs of impending attacks, and thus enabling appropriate responses. You can view this as an “offensive” solution since it might very well require some sort of “second strike” reaction in order to neuter the attack, but most people would nonetheless regard such a response as part of “defense”. And “a huge surplus of distributed offensive power” is of little or no consequence if the equilibrium is such that the “offensive” power can be easily countered.