Air defenses do not need to be 100% effective to be... effective.

Russia cannot keep Ukraine from targeting their oil infrastructure, yet here Russia is, still fighting on. Ukraine cannot prevent Russia from targeting their energy infrastructure or apartment buildings, yet here they are, still fighting on.

If we're talking about strategic/civil air defense, then you must figure out what's tolerable to your population (and how to increase and maintain that tolerance), and then figure out all the means to reduce the incoming attacks to below that tolerance. That must include the full spectrum of offensive, counter offensive, defensive, and informational options.