What I mean, is that it does not really matter that Canada is not the highest paying country in the world, because money is not the only factor in determining where academics choose to live. If we can offer an environment that is conducive to intellectual work (low financial stress, spiritually uplifting environment, a culture of mutual assistance, minimal neurotoxins in the food, water and air, good schools for their kids, freedom of thought and expression, minimal discrimination, no threat of war, etc) then academics will try to come here despite the fact that they won't be as financially wealthy in the global market.

We can offer many of those features simply by leaving the huge local ecosystems intact and accessible, and by avoiding changes that cause parts of the population to become desperate.

That's a roughly steady state flow, all else being equal, and wouldn't explain the change.

I guess both US and Canda pay simliar to postdoc, even in China, because all of them are low.