It's more dangerous because it's more used. Not because of the actual substance. You're way more likely to overdose on heroin or cocaine, as they're both good at building up tolerance - making it difficult to gauge how much to use for the rush you want. Alcohol in combination with other drugs is often a horrible mix, though. Cocaine + alcohol is basically Colombian roulette.
I agree on every point. The thing is though that cocaine and heroin still have horrible stigma, and alcohol doesn't. Even if they were to be legalised within some limit, most people wouldn't want them because they know about the effects. Alcohol on the other hand is so accepted in our culture and so many people consume it in ridiculous amounts that it's still going to kill far more even if Cocaine and Heroin were legal.
On the other hand, junkies will benefit because the cocaine/heroin being produced will be cleaner, more refined, less dangerous and most importantly we'll be able to legally research ways of changing the drugs affects, or finding chemicals much like nicotine patches that help addicts get off the drug(s).
Having said that, all the things we know about how to make Tobacco healthier haven't stopped the cigarette companies from producing them the way they do, using radioactive fertiliser and shipping them out the day they're ready- even though we know the use of this fertiliser increases cancer risks, and that if you leave tobacco to dry for long periods of time (Like pipe tobacco, usually aged in a warehouse for over a year) you can greatly reduce the amount of nicotine in the substance, making the tobacco far less addictive.
So then again, maybe we shouldn't allow companies that might manipulate the drugs' addictiveness to pedal heroin and coke. Aah these issues are so complicated