Ever tried buying pot with anything other than cash?
A very conservative estimate in New Zealand is that the turnover in the marijuana industry is roughly $366 million, or more than a third of a percent of New Zealand's GDP. (It's actually way bigger than that, and there are many other drugs)

And there are many other things that can replace cash, like silver coins which are a far more solid currency anyway. If somewhere between 5% and 50% of most economies is black market (depending on the country) I wouldn't stress too much about cash disappearing!

http://fortune.com/2015/02/18/marijuana-markets-biggest-world