Drug use will almost definitely sky rocket if drugs are legalized, at least initially. We can't really say what will happen, say, 5-10 years after, because legalization in this country is unprecedented.
But ask most users if they'd use more if it was legal, and you'll get your answer. Ask some fence sitters who are too scared to try drugs if they'd use currently illicit drugs if it was legal, and you'll get your answer
Read up on Portugal, they legalised all drugs, just over ten years ago, and the reports released a few months ago (after ten years of it) showed that there are less people using the drugs that previously were illegal (from weed up to heroine), and a higher percentage of those using drugs are in treatment to stop.
Part of the non-legalization problem is that if forces you to go down dark alleys which are certainly not compatible with the all-is-good, geek experimentation philosophy. If forces potential users to associate with the hardcore underground, with all the risks that might entail. On top of that, you will never know what you bought. Testing kits aside, and the futility of trying to scam people on dirt-cheap to produce substances, I would have trouble swallowing anything I wasn't confident I knew it was what it said it was.
Make LSD and other drugs of similar effect legal. Have the government regulate the distribution. Put an exorbitant price tag on them and disallow buying-in-bulk. Make me sign that I am wholly responsible for what happens to me by using them (even though nothing adverse as lung cancer will, I'm sure). Give me a piece of paper that says the LSD I carry is legal. Done.
Where i'm getting at is that I (for various definitions of I) would gladly pay, say, 1.000 USD/EUR for a "trip" if I knew I wasn't illegal or about to get poisoned. Heck, it's even cheaper than non-spiritual trips.
Additionally, you'd likely create a black market for whatever piece of paper says the LSD is legal.
How about a reasonable price tag?
Or how about blanket legalization of cultivation, possession, and use? I can get a tube of spores for "magic mushrooms" for about $10 and grow my own indefinitely. But for some reason this activity (the growing and using part) is currently highly illegal.
I don't need a license to brew beer and drink it myself or give it away. Why should it be any different for plants and fungi?
In the US you do
Very dumb people. That's almost sure to earn you a visit from the least harried law enforcement group in your local area.