Obviously I can't prove a negative and I can find numbers that show that decrimilization caused a decrease or an increase in drug use in the Netherlands. Some is suspect and obviously formed to drive an agenda, some looks to be independent and has been subjected to peer review; pick your numbers.
On the other hand I can't prove that the war on terror has prevented another domestic terrorist attack.
But I'm not willing to give up on the WoT. Do I think that in some areas it could be waged differently? You bet, but do a 180? Nope, not willing to roll the dice.
Here's the difference: the WoT creates no incentives for terrorism, whereas the WoD does create incentives for crack neighborhooods by increasing the relative benefit for users to congregate.