Feasibility of a genuinely reliable system isn't the only issue. Fallout, nuclear and otherwise, is another. Proceeding in this direction effectively would terminate the 1972 arms treaty with Russia, which forbids missile defense deployment. Russia and China both regard a missile defense system as a threat to their well-being, since it would allow the U.S. first strike without penalty. And so they will be compelled to come up with a new generation of missiles that best the defense, which will prompt us to create a new generation of defense, which will trigger a new generation of missiles, and then you have an arms race more dangerous than any threat which now exists.
Money is another issue.
That would be money worth spending if it would buy a guarantee against the nuclear threat, now and into the future, but it is a foolhardy expenditure for a system that may fail and whose pursuit may trigger counter-efforts that should make the U.S. uneasy. In arguing for missile defense, Bush cited neither Russia nor China, but unnamed rogue states whose leaders don't care about their own people and, therefore, would not care if a strike against the United States produced massively lethal retaliation. Moreover, to suggest that the presence of tyrants is a new circumstance which renders the doctrine of mutually assured destruction obsolete is to forget that Nikita Khruschev was a nuclear-capable tyrant and the deterrence principle worked with him. The United States faces serious new security concerns, among them biological weapons, chemical weapons and high-powered suitcase bombs, particularly in the hands of terrorists. The National Intelligence Council, after lengthy study, highlighted some of these in its December report.