This is actually a really old problem since the days of "Star Wars" and the Cold War. It is an order of magnitude cheaper to produce a ballistic missile or low flying cruise weapon with 10 decoys than the requisite amount of interceptors. There is hope that lasers and rail guns can eventually change that, but it's still an unsolved problem.
Defense in the age of missiles and drones is always harder than offense. The only response is to destroy the attacker's weapons before they can manufacture or launch them.
