I don't think it is orthogonal. The US can pretty reliably shoot down ICBMs. Not perfectly, but well enough at this point that if in some extreme scenario Russia actually did launch a barrage of nuclear ICBMs, it wouldn't necessarily mean total destruction. If they can defeat the interceptors though, the risk of annihilation is greater. And it does seem smart to me to put significant effort into combating even small risks of annihilation. IE, don't rely on the second strike deterrent alone, as that may not be foolproof. (History shows it's come close before, plus you could imagine scenarios with an unstable and/or suicidal foreign leader with nothing left to lose.)