The idea that the 407 has oodles of capacity is a fallacy as well. There is a bit of capacity, sure, but the core sections of the 407 are probably already at level of service "D" at peak periods in the summer. Too much more traffic is going to make highway operations unstable. Particularly if that traffic is heavy truck traffic. Heavy truck traffic has much more of an effect on highway capacity than normal passenger vehicles, and too much traffic would negate the "express toll bypass" function of the 407. If that's true today in 2021, imagine how true that will be in 2041. Particularly if the region adds a few extra million people and doesn't build any additional highway capacity.
According to Statistics Canada, in 2016 (the most recent census data), about 24% of trips to work in Toronto's CMA (which would be the highest in the GTA) were made on transit. If by some miracle, Toronto could get 30% of trips to work via transit by 2041, we would still need additional roadway capacity just by the number of additional residents added to the region during that 20 year period.