You're missing some of the history here. Canada's initial free trade integration with the US in the late 80s was controversial at the time, with opponents specifically predicting a slow erosion of sovereignty until one day Canada is forced to subordinate itself to the US. What Trump showed is that that concern was correct, although the erosion was fortunately not yet complete enough to force Canada's hand. The Canadian people don't want to be continually dependent on the goodwill of future US Presidents; they want "Canada should the US" to sound like "Taiwan should join the US" or "France should join the US", an obviously impossible idea that even the most vehement partisans would have to explain away rather than trying to make it happen.

As an addendum to this post, this debate between Turner and Mulroney over (what would become) NAFTA captures some of the feeling of the time [1]

[1] https://www.youtube.com/shorts/jlvb9czZFXw

[2] https://www.c-span.org/program/international-telecasts/canad...