enormous problems caused by prohibition
And...
smaller problems caused by whatever marginal increase in usage repealing prohibition causes
See, the problem is that there is no evidence to support the proposition that the increase will be marginal, and that the harm will be less. All of the arguments rest on the retelling of the 1920s experience with prohibition and essentially say "well we had all these problems with gangsterism in that period, but look, we made alcohol legal again and all the alcohol related problems went away".
There is a *lot* of research suggesting that the overall social problems caused by alcohol are vastly understated, itis just hidden because it happens at the family and individual level. The social symptoms of alcohol just don't make for interesting reading or spectacular Hollywood movies the way prohibition rackets do.
Finally, comparing the ability to enforce a law in the 1920s to today, a century later, is just folly. The circumstances are wholly different. The real problem underlying the ineffectiveness of today's drug "prohibition" is a lack of political will, conflicts of interest and outright corruption.