If cannabis were to be legalised I don't believe it would stop the illegal trade for the sole reason that the government would greedily tax it to make it ineffective from a cost perspective. It's what they have been doing on alcohol and tobacco for decades. Around 80% of a packet of fags is some form of tax.Also, drug laws are changing around the world so there is plenty of real world evidence to now look at. The US is now at the forefront of the weed industry. German laws have just changed. If the puritanical US can change things then so can most western nations.
For the record I am not a huge supporter of drug legalisation, more someone who simply sees the failure of decades of war-n-drugs and does not see the sense in keeping it illegal.
That's why so much illicit tobacco and alcohol gets smuggled in and is so readily available on most high streets.
Yes, legalise cannabis but if the government taxes it we will swiftly be back to square one.