Like you said, I think the returns will go to the top. In 50 years, I would expect the biggest metros will be Toronto, Montreal, Vancouver, Calgary, Edmonton - in that order. As I mentioned in another thread,
9 of the 10 largest cities in 1921 are still the 10 largest CMAs in 2016, including cities that we associate with rapid, postwar growth like Calgary. They're not in the same order, but they're all on the list. That's definitely not the case in the US, where large cities in 1920 like St. Louis and Cleveland are way down the list, replaced with Sunbelt metros.
Canada is not like the US where there's a lot of permanent internal migration. The exception is resource workers from Atlantic Canada and Newfoundland, but there are only so many of them. There won't be a giant city emerging out of nowhere like Phoenix or Tampa.
That said, if I were to bet on a city that will be big in the future that's not a bedroom community of a major CMA - like Barrie or Abbotsford - it'll be Red Deer, Lethbridge or Kelowna.