i don't think complete invulnerability was ever the goal of missile defense. It was meant to be a countermeasure to something where before there was none. I'm actually surprised it works as well as it does. Back when these things were first being developed and tested the thought was intercepting nuclear armed ICBMs, they were supposed to be massively destabilizing with respect to MAD and could conceivably give a nation first strike advantage. First strike advantage means just bare minimum survival not that you never get hit at all. Fortunately, that never really materialized.