I'm saying it already happens. I grew up with people who literally smuggle weed for a living across the northern border. There are some areas where there is no actual official border[0] where smuggling is happening now. It was already comparatively easier to access in Canada then smuggle to the US, but when it's legal in Canada it will just help fuel the supply being brought over.

[0]https://en.wikipedia.org/wiki/Akwesasne