Legalizing pot is a great first step to see if legalizing other drugs would work. It's the least harmful of the bunch, and the one with the least social stigma attached to users.
You're thinking isn't bad. But it will ultimately fail. Here's the problem: You believe in a negative correlation between the lethality of a drug and the social value in its legality. When the truth is the reverse. The more deadly a drug, the more important it is to legalize it, and even to use public funding to dispense it.
Sometimes the best way to prevent a forest fire is to let the bushes burn themselves out.
What I believe is that the current system isn't working, and that something else has to be done. Hell, right now smoking cigarettes holds more of a stigma in some circles than pot does. A sizable number of people view laws against pot as something of a joke, considering the # of people who routinely ignore those laws.
For harder drugs, I would wait to see the results from pot legalization before deciding on any others.
And your solution of letting people just off themselves with said harder drugs is a no go for obvious moral reasons.