I think gf0012-aus is hitting the nail on the head with combined arms. If you have only one arm in your defence, it can be circumvented byt the enemy.

I think swarming makes a lot of sense. Saturating an airspace with lots of simple, low cost, reduced signature uavs that simple hunt for an aircraft not returning friendly iff. You could "manage" the swarm from a standoff distance, painting civillian or friendly aircraft, or high-priority enemies with some kind of indentifier; changing the swarm's ROEs. Also, I think passive detection is the future -- I think you'll see air defense and air warfare look more and more like submarine warfare. Arrays or satillites that can optically scan large swaths of airspace, or that use sound or other emmissions to find and identify aircraft will allow many air defenses to operate without giving away their positions -- suppressing SEAD. Ultimately, a combination of passive detection and swarming could make a future battle between two advanced nations an amazing spectacle -- thousands of stealthy, small uavs floating through enemy airspace, being detected by a passive sensor which then relays the launch command to a remotely located missile. Each side attempting top stay hidden, and then "poking it's head up" to try to get a shot in. Very much like a submarine battle.

Interesting points... still UAVs have not yet proven themselves enough as combat platforms. That may change in the future, but if I were the head of a large airforce, I would hedge my bets. The prevalence of passive sensors does, however, seem like the way forward for air defenses.