Alcohol consumption did drop during Prohibition. What making alcohol legal did was exchange one set of problems with another. When it was illegal, you had a violent, vicious black market and people dying from bad booze. When it is legal, the violence shifts into the background of society - into individual homes, into cars - the violence of DUI car crashes, of booze-fuelled domestic violence and child abuse, due to the far greater availability and lower cost of legal booze - casualty numbers far greater than that of the black market, but much less flashy and therefore far less noticeable.
The same is the case with hard drugs. So it is absurd to pretend that legalizing drugs is a solution in its own right - instead, it replaces one problem with another. And should not be glibly proposed as a result - instead, there needs to be some kind of middle ground, knowing that there is no good answer here, only bad and worse.
Realize that when someone drives drunk, in most cases, they make it home OK. But the consequences of failure when driving drunk, and the much greater likelihood of failure happening when driving drunk, necessitates making such an activity a crime, whether or not violence happens as a result of that activity. Hard drugs fit that pattern as well - the very high likelihood of really bad consequences radiating out beyond the individual consumer of the drugs.
It is easier to deal with the other issues sans a hail of bullets.
"Love the people for what they are,not what you would have them be";old advice but still relevant.