While I whole-heartedly agree with you, it's easy to frame this situation in a misleading light: "we've been fighting drugs for 50 years, and we still barely have them under control. Imagine how much worse the situation would have been if we hadn't fought them! Now give us more money and let us be even harsher!"

Of course, this is pure sophistry, but it will appeal to many people. The better line of argument I believe is about the extreme positive impact that decriminalization/legalization have had in every single country that have tried. That is much harder to dispute and twist.