It is divorce in anticipation of a bout of domestic violence.
Simply eliminating a lot of those domestic trade barriers would create more economic wealth than what Canada would lose by ending trade with the U.S. completely.
Of course in practice it won’t be that easy and the finances don’t usually materialize that easily, but the point is Canada has options for growth that are fully under its control.
Inter-provincial trade barriers for labor, especially licensed labor are also quite onerous. But it's still easier for a Quebecois tradesman to work in Ontario than it is for that same tradesman to work in the US.
I am in the start up community in Canada. I can tell you that after the first threat from Trump every federal program to help tech start ups immediately pivoted to Asia and the EU. Before he started yapping, we were connected to Canadian representatives in the US, meeting about markets and opportunities. Now all programs are directed at forming partnerships elsewhere.
It's certainly not guaranteed, but taking an aggressive defensive stance is the ONLY possible way to stop having your lunch money stolen.
You don't reason. You remove the victim from the hands of the aggressor.
It will cost a lot of money, and the Canadians will suffer greatly. But the alternative is to join America, which Canadians have stated don't want to.