Cross country comparisons are difficult. Just because Dutch consume less marijuana than U.S. it doesn't mean that U.S. will not consume more when it's legalized.
As far as I know, there aren't any records of a boom in consumption just because a drug was legalized. And I'm not talking about marijuana only, but drugs in general, alcohol included.
Considering that, we can say that the fear of people going mad about a drug just because it was legalized is another myth of the "war on drugs".