No defence weapons systems can be said to be successful until it has come under sustained attack from malevolent influences.
The French believed the Maginot Line to be impregnable.It may well have been, however, it was flawed in that it could be bypassed,and was, by the Germans attacking through a third country.
Blocking missiles may prevent that particular method of delivery, however the kind of organisations that currently represent the biggest threat to US domestic security do not have access to such technology. It only takes one missed suitcase bomb in a crowded location to completely negate the effects of billions of dollars of high tech gizmos owned by the government. Let's face it, the U.S. was spending billions on its military in 2001, and still the system was beaten by a bunch of Saudi fundamentalists with minimal weapons.Rather than spend telephone numbers on big shiny weapons, the U.S. could be better off splitting its defence budget between public relations in the form of aid to difficult regions, and enhanced intelligence to identify and neutralise potential threats.A missile system will cost a bizarre amount of money to treat one symptom of the problem for a short time.Technology is such that it would only be a matter of time before countermeasures rendered it obsolete.If the technology exists for a fighter plane to detect hostile weapons being aimed at it, those systems could easily be reconfigured to work on a hostile missile. What's to stop someone developing a 'dirty missile', designed to release its payload in a thousand smaller warheads if it detects a defence system launch?how would the new system cope with a thousand golf-ball sized grenades filled with radioactive material or biolocical agents? History is littered with obsolete defence systems.Spending the money on eliminating
the breeding grounds for hatred could provide a more lasting solution.