The government is always looking for ways which they can pork barrel by American MIC while appeasing Washington, so joining the American BMD is an option they are seriously considering, they are just afraid of the reaction which will come from the Anti-American Peaceniks.
The new Canadian warships, if they ever get built, will have the Raytheon Air and Missile Defense Radar and would be able to load the Aegis BMD RIM-161C mid course tactical ABM, so the option to go with American BMD is provided by Lockheed Martin Canada, should they decide to arm the ships with the ABMs.
I can't see them buying THAADS, single role system doesn't do anything else, so for land based BMD, I would say they would just allow the Americans to put their BMD systems in Canada, plausibly in Goose Bay Labrador which is a base which has no use, but which the government would like to keep open for vote buying purposes.
The Canadian military doesn't bring much to the table anymore, so BMD would be a way the government could get more bang for their buck, for the purposes of sucking up to Washington with something which is actually relevant to Washington.
The problem in Canada is that only the navy and air force are of much use, but all the budget gets sucked up by the army supposedly to do "Peacekeeping", but all we end up with is no navy, no air force, and an army which can't actually fight wars. BMD would as least be something, rather than what we have now, which is nothing.
They cancelled the F-35, so Canada is going to be out of the air combat business, they won't buy new submarines, so Canada is not going to be defending the continent that way, the Type 26 with BMD is basically all they got to offer, again, if that is not cancelled too when it inevitably goes way over budget.