Yes, we tried to start yet ANOTHER insane arms race with the Russians over this, but internal collapse put an end to it, at least for a couple decades. The problems of basing any rational geopolitical strategy on trusting that your anti-missile system can stop EVERY one of the other side's re-entry vehicles is a kind of insanity we fortunately avoided. The obvious downside is that missing even ONE RV ends up losing an entire city.
No one actually knows what will happen to a real city with one of the high-tech nuclear devices, as nobody has used one against a real city, thank goodness! But, modern two-stage weapons are WAY more powerful than the primitive single-stage devices used in Japan, so it is pretty clear it would be SO much worse!
Nowadays, it isn't highly sophisticated rivals that we have to worry about, it is a tramp freighter with a rogue cargo container steaming into some port. Homeland Security is working on that, but it is a pretty tough problem.
Jon