I'm not an expert, but while hemp in general might be a fairly hardy crop, the strains of cannabis prized for their cannabinoids might be a little more finicky. Hemp can endure a wide variety of molds and diseases and keep growing without losing much yield, but this might be less of a virtue in a crop destined to be commodity-graded for human consumption. Also, I'm given to understand that the intensity of the sunlight available to the plant is a major factor in determining the potency of the final product. I would be surprised to learn that any of the excellent marijuana grown in Canada was planted outdoors.
Combine that with how responsive the yield and quality of harvest is to intensive cultivation, and it seems a more likely competitor for greenhouse space in Niagara than for acreage around Tillsonburg.
Will they? Or will the marijuana farms just move outside, in the open since they no longer need to be hidden.
Pot isn't hard to grow (I'm told), it's just hard to hide the fact that you're growing it.