Are you saying the 10th Amendment reserves to the states the authority to legislate on "soft" drugs but not "hard" drugs? Where in the language of the Constitution do you find that?
Libertarianism relies on the assumption that people will be highly responsible.
No, it simply advocates that government not shield the irresponsible from the natural consequences of their irresponsibility.
The problem is that libertarian policies lead to societal degradation and degraded societies cannot support libertarian governments. After one generation of liberal drug policies, this country would be run by communists. So many people would be so reliant on the government
You assume that a large number of people would not only use drugs but use them to the point of inability to support themselves. Why should anyone believe that? Are there that many people who are deterred from drug use by its illegality, but who would not be deterred by the risk of addiction and degradation? Is illegality YOUR sole reason for not using drugs?