This is amazingly naive and 180-degrees wrong. Once the highway is built, you've permanently affected the environment and there is no going back. "Can easily be demolished?" You must be kidding. But if you have some free time, check out the wikipedia for a little road called "The Gardiner Expressway," and see what you find.
You can believe that 413 will offer benefits, but that's not a "fact" anyone is "ignoring."
I also don't believe you get "the same amount of sprawl," given what we know, and have long known, about the fundamental connections between urban growth and transportation infrastructure. The lack of highway makes these places less accessible, less attractive to purchasers (certainly those for whom it would have been a relatively easy commute to the City) and therefore less profitable for the developers to build on.
This is the Growth Plan map, with the highway corridor shown as a pinkish arrow...
View attachment 317173
...certainly at the eastern end, it turns through a big purple area that will grow but then it goes through the greenbelt and a big chunk of nothing. So, aside from that Vaughan/Brampton area, I don't see where you're getting lots of sprawl without the highway.
Developers buy land and hope. Sometimes it gets put in a greenbelt or otherwise taken out of their hands. That's life in a capitalist society. Just think back a few years to the Vaughan developers who got DelDuca to advance the Kirby GO plans; they KNOW they need that connection to make their development work. The fact that it's owned, does not mean it's developable.
And I understand the pragmatic argument that we're growing and people have to go somewhere etc.; there's always a push and pull. But the denial of this corridor reinforces pushing intensification growth. There's still lots of room in the urban area, particularly around all the transit infrastructure being built. Time will tell how it all shakes out and perhaps it's simple to assume the death of the 413 means there's no growth "out there" but I also don't buy you're just going to get a lot of sprawl that's super inefficient because there's no highway.