your arguments sound pretty persuasive on the surface. BUT... a few questions:
If drugs are legalized and prices drop, won't there be more users? The theory on tobacco use, which is legal and expensive to society, is that if you raise the price there will be fewer users. Perhaps the converse is true for drugs. As you said, drug users destroy themselves. I suspect destruction would be more widespread with readily available drugs.
If the elements of our society that profit from drug trade are put out of business by legitimate distribution sources, how will they maintain their standard of living?
Which drugs would you legalize? Just marijuana? Hash and coke? Where do you draw the line? Won't there always be more powerful illegal drugs tempting a greater number of recreational users and addicts?
Most of our social problems come from irresponsible exercise of our precious freedoms. Legalizing drugs would drag us down further.