There's really no such thing as an indefensible missile, just like there's no such thing as an impenetrable computer network. Difficult to defend against, those exist. Missiles with an extremely low probability that we can physically intercept, those exist. But defending against a missile starts as soon as a possibility of launch occurs, and you can bet that a lot of people spend a lot of time figuring out how to defend against "indefensible." They aren't perfect, but they aren't going to bat .000 either.