I was thinking about that issue and remembered a conversation my brother and I had several weeks ago. I said something about Russian long range air to air missiles, and he pointed out you know longer that you've been targeted and have more time to evade. That's when hypersonic missiles made sense ... until you look at the other side of the equation. The faster they go, the less easily they can maneuver, and the more predictable the trajectory. Whether you look at it from a velocity or momentum standpoint, as velocity increases, it becomes increasing harder to alter and correctly alter the path to track an aerial target. It also would mean an air to ground missile would be more predictable for an intercept. The keys to intercepting a hypersonic missile would appear to be prompt detection and being close to the line of travel - you aren't going to win a tail chase.