I don’t think that old plan would work today. Take severing undersea cables. Some of the major Pacific undersea cables terminate in Vancouver, serving the entire Pacific Northwest. Severing them seriously degrades communications throughout northwestern US while not disconnecting Vancouver from the rest of Canada, or the world at large.
Not to mention: how exactly would the military hold these strategic cities? The majority of important/export from the port cities is already to the US. If they tried something like this, Canadians would just go on a general strike and the US would be severely impacted.
That’s assuming of course that NATO ignored the situation and China didn’t take the opportunity to invade Taiwan and Japan.