There are a couple of overlooked points in this discussion, for example
(1) Granted we need high levels of immigration to maintain (even remotely) the ratio of workers to retirees we have enjoyed to date. But is this even possible? Are we even going to be in a position to attract half a million people a year? Canada is not the only country in the world facing a demographic crisis. At least tweny countires are now experiencing population decline, including most of the European nations that are able to supply the educated people we are seeking. Many African nations are generating excess population, but these people don't usually have the eduation levels Canada is looking for. And no, this has nothing to do with "Canadian racism". Canada is NOT, in general, a racist place. Really, it's not;
(2) In spite of what we like to imagine, many people do not consider Canada all that great of a choice, and our immigrant retention rate is on the low side (lousey cimate, taxes too high, nowhere to live, lousey food, fat, unattractive population, the impossibility of getting foreign credentials recognized, well, you get the picture);
(3) We may very well have to resign ourselves to working longer and living on less (well, not public employees and politicians. They deserve more that the rest of us for the great job ther're doing). And yes, I'm speaking from expereience; I'm seventy-three and I still run my business on a full-time basis.
(4) As many have pointed out, Japan is the test case of what happens to a declining population that refuses immigration. Well it looks bleak for that island nation, but they do have two advantges that we do not: the people accept working into old age (they realize retirement is often destructive), and they are a vastly fitter and healthier population. Perhaps we should look along these lines for our future solutions to the demographic problem.