I think people need to research what MIRV technology is. The interceptor can only contain one missile while the incoming MIRV contains multiple warheads. Besides, decoys will be used so the interceptor simply can't know what warhead to go for. This is totally a massive game changer. To date no country has demonstrated an ability to take out MIRV technology, if I am not mistaken.
There are MIRV-killer warheads too, Falcon. Like the American Multi-Object Kill Vehicle, designed to fit an SM-3 interceptor. This is an older version. The new version is still classified:
Penetration aids like kilometer long chaff spools, decoy or dummy warheads, Mylar balloons, radar jammers... they're all tried and test methods and they work, but they aren't fool proof and advanced interceptors have the means to discriminate between live warheads and dummies via various metrics such as heat, weight or filling.
That said, most interceptors designed to knock out MIRV missiles try to do so before separation, to make an interception and total destruction of the missile easier and more complete. GBI tries to do this at ranges upwards of 3000km and beyond, long before the MIRV separates from its bus.
MIRVs and MaRVs, as found on the retired Pershing II:
Complicate the process, but don't make the missile infallible either. There are countermeasures and tactics available to a nation to defeat MIRVs.
