Maybe, but you're not considering (a) the value of the target or (b) the effectiveness of drone defense. Even if defense is cheaper than offense, it will never

Maybe, but you're not considering (a) the value of the target or (b) the effectiveness of drone defense. Even if defense is cheaper than offense, it will never be perfect. If all it takes is one drone to destroy the target and the target is sufficiently high-value, then the 10,000 drone strategy may be worth it. For comparison, see ICBMs and missile defense.