It is impossible to say how much alcohol consumption was effected by prohibition. Obviously normal measuring methods were not around, Al Capone wasn’t paying taxes on what he sold. If you use cirrhosis deaths as a proxy, prohibition decreased alcohol consumption by between 10-20%. Obviously not a perfect measuring stick. For what its worth, despite being legal, alcohol consumption has been going down since the 80’s. Google it. You'll see the truth. Alcohol consumption went down dramatically during prohibition and increased after prohibition. According to Encarta, per capita consumption rose from 1.2 gallons in 1935 to 2.7 gallons in 1975.
Now why would you not expect the same thing to happen with pot?
Show me your statistics on Amsterdam.