why not?It's not really possible to have an effective shield that would have a perfect success rate against a strategic strike from the likes of Russia, America, China, Britain or France. You could potentially intercept missiles fired by the likes of Iran and North Korea, but they'd very quickly be satured by MIRVs equipped with counter measures. These missiles don't go in a straight trajectory for instance, they do circles and spirals and are going at many times the speed of sound, plus they have active counter measures, and there would be dozens or hundreds of them. I mean a submarine could fire them a couple of dozen miles off the coast or something, good luck defending your entire air space and coast line.
you realize that we built all those every expensive and rare material filled MRIVS, we could build a hundred or even a thousand times as many interceptors.
and that's assuming we dont develop say a rail gun, or energy based interceptor at allows rapid fire from ground, mobile and air based platforms.
or hyper sonic missiles capable of intercepting launch vehicles etc
to say we wont ever advance past cold war technology is very silly indeed.