I completely agree that the line is the dangerous part, and it's an unclear target to aim for! I think that's exactly why we don't see major strikes against nuclear powers. Nobody has a public policy stating "we won't nuke you until exactly X, Y, and Z happen" so everyone else is left guessing. This is far from a complete deterrent, which is why we still see probing attacks like Solarwinds, but they do not strike me as decapitation attempts.