Well, history teaches us that it probably is the case. During the alcohol prohibition in the USA, the situation was more or less equal than before it.

There aren't hard numbers of consumption during the prohibition (obviously) but statistics of alcohol related deaths and arrests seem to show that there was a serious decrease just before the prohibition, and after a few years it returns to its previous state, and was worse in some aspects.

http://www.druglibrary.org/prohibitionresults.htm

Also, in the Netherlands the marijuana consumption rate is less than in the USA, despite it being almost legal.

So, don't worry, things won't change much, except for those people who won't have to be worried about being arrested for smoking a plant.