No defense system is 100%, and SDI was never intended to be. But if you are in one of the cities that does survive an attack because an incoming nuke is intercepted, you will be more than grateful for the money spent.
Let’s see...$50+billion for SDI...what’s the cost of just one major metropolitan area being hit, let alone several or many? By the numbers, the system pays for itself very quickly in the event of an attack.
SDI never worked to begin with.