Great post indeed!It's very hard for cities to overtake each other when they get to the top of the lists for their national population. Being one of the largest cities has significant (although not insurmountable) advantages. Assuming growth continues overall in the country into the long-term, it will likely continue to accumulate to the larger cities disproportionately over smaller centres. For all the talk of Montreal's "decline" it is a relative decline, not absolute. Montreal's CMA still added nearly 1 million over the past 20 years.
I think it is conceivable for Calgary to continue to gain on Vancouver proportionally, although I wouldn't say it is likely we will ever pass it in size. Maybe? But not sure if there is much long-term evidence to support such a claim. It also isn't clear that anyone smaller than us will clearly surpass Calgary, although Ottawa and Edmonton are similar enough in size that inevitable fluctuations in the local economies and demographics might be enough to trade around the 3-4-5 spots for the next century. I would be interested to see how the Calgary-Edmonton corridor evolves on such a timeframe, Canada doesn't have another example of 2 centres of 1 million + only 250km apart both with large absolute growth potential. We are already well linked economically, and that is likely to continue. Perhaps a 5 - 7 million person corridor (~3 million currently) in 50 or 100 years is in the cards! That could be exciting.
Predicting the future is fun!
It's definitely interesting to think of Calgary and Edmonton as the anchors of a future "megaregion." I don't know if that's a technical term, but I end up floating back to that wikipedia page every so often. It unfortunately tends only to outline developments going on in the US, but the Calgary-Edmonton corridor has the potential to be a similar future all-Canadian region. The Calgary and Edmonton CMAs are already rapidly approaching the 1.5M mark and by next census the region should have it's 3rd CMA with Red Deer.