If all the Canadian Captains would have to vacate their US bases and these 45 slots would be made available for bid, most would be not filled.
Most Americans at CX are not senior enough to hold a Passenger Command. Proof me wrong, but last time I checked there are not too many Americans with a seniority # in the high 900s. As we all know all new command positions are within seniority offered in Hong Kong. Only captains or when commenced the command course one is able to bid a base slot.
In reality this means that most of these US positions could not be filled if offered and these flights would be crewed with Hong Kong crews.
Plain and simple.