I don't think they can simply block the trade routes - surely people can route around it. In any case they protest against not being allowed into the country with their trucks, so either way the trucks wouldn't be running?
The Ambassador Bridge links Detroit to Windsor across the Detroit River at the southern tip of the Ontario Peninsula. The closest US-Canada land crossing is the Blue Water Bridge between Sarnia, ON and Port Huron, MI, 2 hours' drive north.
At least it can be rerouted. I don't know - I am not well versed in "protesting philosophy". The economic damage seems less serious, after all, the truckers could also just decide to not come to Canada to begin with. It is the mandates that make it difficult to enter that do most of the damage.