Increased healthcare spend: But why don't we draw a line for alcohol and tobacco? Or people eating too much? All of these are legally sanctioned choices people make. It gets into slippery territory when you start telling people what they can or can't do because of the burden they'll place on the healthcare system. Slippery territory that leads to the situation we currently have.
People stealing your tv: Spend less money on chasing and locking up drug users and more money on chasing and locking up burglars. Also, if we're just talking about simple economics here, the cost of supplying the entire world with high quality, low price drugs would not be terribly high. It's like, "How many homeless alcoholics are robbing homes because they can't find a drink?" That could be paired with a general public education campaign about the realities of drugs (I mean, Trainspotting and Leaving Las Vegas are kind of about the same thing right?)
So I deny that there are negative societal externalities. I'll make an exception for the fact that families pretty much literally lose loved ones to addiction, and that's a pretty big one, but I think it gets into the kind of territory of "do you owe your family anything?" I would say no, if you want to throw away your life to drugs, that's not the state's business.
And again, I'm really not advocating anyone start doing any of these things, I'm advocating that the state stop locking people up for doing them. In order for that to happen, it requires a certain amount of acceptance.