And you only have to look at Russia to see what happens when supply completely dwindles to addicts of hard drugs, the krokodil epidemic is truly horrifying.
My point is just that a free for all on all drugs no matter what harm they cause could be disastrous for society as a whole, as you mention there are plenty of people that currently use alcohol to escape their problems that are in difficult places, especially in poorer demographics. Imagine if they could get hold of heroine or methamphetamines on a whim.
It's a fine balance and I personally cannot see a good way to strike one that will keep everybody safe and happy.
There are plenty of drugs which if used responsibly I could see being legal though, marijuana, ecstasy and some hallucinogens to name a few. If taxed the government could stand to make a lot of money from them as well.