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Many drugs have been made either outright legal or de facto legal (you can do business un-harassed in many open air drug markets today, and people wander the streets obviously high and nobody does anything). Yet gangs persist, in fact they are more common in those districts with lax laws and enforcement.

By contrast Singapore puts people to death for carrying too much pot and its got no crime at all.

An interesting thing with prohibition is it didn't spring into existence in 1920. Most of New England and NY and the Upper Midwest banned alcohol in the 1850s (it was repealed during the Civil War to raise money for the war effort).

Throughout the late 1800s and early 1900s many states, counties, and localities passed various prohibition legislation long before prohibition. If you looked at a map of the US in 1905 huge swaths of the country were effectively already under prohibition, but the crime rate was low, and it was lower in the dry areas then the wet areas. Most gangs were in the big cities amongst immigrant communities, that were thoroughly "wet". Gang and criminal activity was on the rise from 1900-1920, before prohibition, and mostly in the wet areas. The murder rate increased by a factor of 600% from 1900-1920, and only 66% or so during prohibition.

I generally think that prohibition was a net negative:

1) It tried to ban a "soft" drug with deep roots in the culture

2) The ban itself was harsher then many of its supporters expected (most people thought prohibition would ban alcohol content above 3.0%, light beer level, and that it was mainly supposed to prevent binge drinking of hard liquor).

3) The enforcement was basically non-existent. There was no plan for enforcement and few resources applied to it. Local governments, especially those in the cities where people voted wet and didn't want prohibition anyway, did basically nothing. The federal response was more or less stillborn from the start for a wide variety of reasons you can read about.

4) There was a rise in crime associated with prohibition, but there was a bigger rise in crime before prohibition. In a way the prohibition era is just a continuation of the trends of immigration (both from Europe and blacks from the south) and urbanization which you would expect to increase crime rates. Before prohibition Sicilian mobsters were finding other ways to commit crime. And places with prohibition but without Sicilian mobsters seemed to go along peacefully enough.

In short, while I wouldn't endorse prohibition, I don't really consider the be all explanation for crime trends in America at the time, and one quick fix to solve it all. Nor do I think that meth or crack being illegal is the primary driver of crime today.