Most 'illegal' drugs are in fact taxed to make them illegal. (You can't get the 'tax stamp' to prove that you've paid the tax, so they are permanently illegal.) Machine guns are also legal and taxed. I haven't heard of any drop in demand for either.
This kinda goes the route of 'weather'. You can talk about it, but there's not much you can do about it.
I guess there was no lesson learned from the 'prohibition' as illegal items will just move to another form of operation and will go on as usual. Nothing changes, except where you have to go, to buy what your looking for.