Like I said. A lack of war will be taken as direct evidence that it works (not any other causes). And the only way to disprove it conclusively is if we all wipe each other out.
It’s kinda like economics. We can’t really prove anything in economics works the way we think it does, but we have a bunch of REALLY GOOD suggestions to support our hypotheses.
MAD isn’t a natural law - it’s a social construct, very much like economics.