If you have a social welfare state, then a lot of people will feel no compulsion to get their lives in order and be productive. Government will spend a lot of money taking care of people who need help. This is a comment on Human Nature and has nothing to do with drugs. LBJ's great society is an aspect of this, and is not tied to drug use.
If you legalize drugs, then a lot of people will find themselves with no compulsion to get their lives in order and be productive. Government will spend a lot of money taking care of people who need help.
If you combine these two things, then things get real bad real fast.
As far as I'm concerned, if you end Social Welfare first and force people to be responsible, then go ahead and legalize the drugs. At that point, people will either die or figure out how to control themselves.
So you think there are a lot of people who currently avoid drugs because they're illegal, but who would under legalization not avoid using to the point of disorderliness and unproductivity? Why should anyone believe this? Are you such a person?