[pinky1417]

It gets worse. If your adversary believes your missile defense system is highly effective, when in fact it is not, the adversary will be more willing to launch a preventive attack. You don't want nuclear-armed adversaries to think they're in a use-it-or-lose-it position unless your system is about 100% effective.

Israel's Iron Dome is one thing. They're destroying small, short-range, generally unguided missiles carrying conventional warheads without decoys. The US's anti-ballistic missile systems (e.g. GMD, SM-3, THAAD, Patriot, etc.) are unlikely to provide meaningful protection against ICBMs even from relatively unsophisticated enemies for one reason: it's substantially easier to build swords than shields. If North Korea builds a missile that can carry a nuclear warhead into the US mainland, they can build the technology to stuff decoy warheads into the payload (eg balloons with the same signature as the actual warhead).

You really don't want a nuclear enemy overestimating the effectiveness of your missile defense!