The events that would cause the extinction of cannabis would also mean the extinction of most life on earth, starting with the top of the food chain [humans and other carnivores...look at the Polar bear situation... the chain of events is already begun].
I believe that cannabis will outlive us as a species. When our atmosphere is too rich in CO2 to sustain animal life, plants will flourish, especially hardy ones such as cannabis... and all the breeding effort that has been put into making it hardier still and able to thrive in most environments on Earth will only increase its chances for survival.