A massive correction is required on your "simple demographics" comment.
Look at the population growth stats for Ontario as posted by the Ontario Government:
https://www.fin.gov.on.ca/en/economy/demographics/projections/table4.html. These stats come directly from StatsCan, the same source as noted in your post.
In addition to the GTA stats (which are incorrectly denoted in the table you posted - the Reference projections are not listed), look at the tables for Hamilton, Brant, Dufferin, Wellington, Waterloo and Niagara (west) and compare the projected population in 2041 with communities to the east, Northumberland, Peterborough and Kawartha Lakes. The project growth in Ontario is clearly to the west of Toronto. The projected population for these communities in 2041 is 2.6M residents west of Toronto vs 263,000 to the east.
I truly don't understand the spin here. Whats' with the dogged support of investing potentially tens of billions in an airport that is clearly not required in this location?