"So has the removal of prohibition removed problems due to trading in illegal substances? Answer, by your own admission, is no :)"
"For your scenario to take affect all drugs have to be
legalized."
"I feel the problem would not go away"
You seem to assume that the amount of illegal "effort" to supply illegal substances (with the violence that follows from those activities) has and will remain constant trough prohibition/legalization of some of the most common drugs (alcohol and cannabis). This contradicts basic economic theory. If there is a bigger demand (for illegal substances) there will be a bigger supply (of illegally distributed illegal substances) if an increase in this supply requires organized criminal effort (involving, among other things, violence) prohibition will lead to more crime. (It could also decrease other forms of crime but thats not the matter here.)
"Essentially what you are arguing is the idea of shifting poisonous assets (crime) elsewhere to allow positive gains in certain areas (cannabis industry/use)." I'm not arguing for anything, I'm trying to get the point above across. There are cost and benefits to prohibition of any substance. One of the most widely recognized costs (this is literally the first time I have hard it questioned) is that it causes organized crime.