I agree entirely with your point about taking supply out of the hands of the underworld. Were I a parent these days I'd be terrified of what young people are taking. 40-odd years ago, much of what you got hold of was probably pinched, but at least legally manufactured. Nowadays, most of it has been banged together illegally and there are no controls over strength and quality.
As for the powders that people take, what has been used to reduce the strength of what gets to the street is any white powder, flour, french chalk or caster sugar if you're lucky. There have been cases where rat poison has got in there.
Few people die from the actual use of heroin. Many deaths from heroin are caused by people not knowing the strength of what they're taking. Paradoxically, it would seem that the occasional dealer gets generous, doesn't cut it enough and the client gets something far stronger than what he's used to.
If drugs were processed by the likes of Glaxo and Wellcome, there would be all manner of controls in place. Also, of course, the government could put a tax on it to pay for the policing.
I'm on about chemicals here, not any form of weed.
There are potential problems - as I understand it, it is only recently that it has become possible to test drivers for the presence of many drugs and it isn't always possible to do so without sending samples away and waiting quite a while.
There is also the possibility of all manner of smack addicts turning up here from all over Europe in search of a cheap source of supply.
The main problem is that no politician will ever dare to allow it to happen, at least not without a long period of trying to persuade the public that they won't be slaughtered in their beds by drug-crazed hordes.