Well, I suppose that would make sense if we lived in a land of infinite resources. As it is, we seem to be short-changing lots of less glorious-seeming but more practical security measures.
And, of course, if we are going to defend against the most unlikely means of attack, it would help if we actually were able to defend effectively against it. While it may be unlikely that NK would shoot an ICBM at us at all, it is even less likely that if they do so they will be so kind as to give us exact warning of when they are doing it, what the are aiming for, do it only in good weather and with a geometry similar to what we’ve tested, tell us what the warhead and any decoy looks like (and make them look sufficiently different)… And, that is what we would need them to do in order to have even a sporting chance of shooting their warhead down with the system as it would be deployed next year.
Even in that world, ABM effectiveness might be compromised by our inability to maintain a high state of competent readiness on the human side of the system. I’ve seen absolutely nothing on how they plan to staff the batteries, or control the decision to launch.
It’s the same problem we had with controlling ICBM sites back during the cold war, and despite assurances that everything was cool, that wasn’t handled very well:
I’m sorry, but this is just not right. Modern ICBMs are pretty accurate, and more importantly, they are cheap. Think about it: what is cheaper to build and maintain: a few dozen ICBMs, or a nuclear sub with a few dozen SLBMs? ICBMs (espcially mobile ones) are the backbone of Russia and China’s nuclear deterrent, and are effective because no missile defense system exists.
Even for the United States, ICBMs remain a very effective deterrent force, because they are so numerous that there is no hope of a disarming first strike. Along with SLBMs, this has a significant effect on others’ strategic doctrines: China, for example, does not seek a counterforce nuclear deterrent, they never have, and it is unlikely that they will pursue one in the future.
There’s even been talk this year of refitting surplus ICBMs with conventional high explosives to serve as a global, rapid tactical strike system. Damned if I can find a decent reference for that this AM though.
And people would be able to tell the difference between a nuclear launch and a conventional launch how exactly? I can see people considering it but the global reality is that an ICBM launch is going to have a lot of people assuming the worst. And likely reacting in the worst possible way.