Stopping ICBMs, is not impossible, just difficult and expensive. Citing the pathetic performance of anti-aircraft missiles misses an important point. Most of the surface to air missiles fired over the last four decades have been Russian. And these missiles have usually been fired at American aircraft. Less capable Russian SAMs trying to high higher tech American aircraft has been a dismal exercise. But when you look at the smaller number of American made SAMs fired at Russian made aircraft, the hit rate goes way up (more than ten times.) So no matter how daunting the task of building anti-ICBM systems, their political popularity, and technical inevitability will continue to out vote the technical and fiscal pessimists.