Personally, I think that the Greenbelt has a valid and extremely important reason to not only be maintained, but strengthened. Not only does it protect the Oak Ridges Moraine, an ecologically important and vulnerable landform, but it provides a necessary impediment to expansion. Vancouver wouldn't be the city it is today if it wasn't hemmed in by the water and the mountains; aside from the lake, southern Ontario has no similar geographical stumbling blocks, so it's up to artificial ones to do their work. Limits encourage people to solve problems and try new avenues to success - whereas in a situation where everything is straightforward and open, the easy choice is going to be taken every single time. But we have to put in hurdles to those easy choices, or what we're going to end up with is low-density sprawl coating the land like a fungus.
An aerial view of part of the low-density Phoenix metropolitan area
The constant construction of new sprawling subdivisions of single-family residential homes is a windfall to the development industry, sure, but they're the only ones who truly profit by it in the end. Today's suburbs aren't communities but hollowed-out zones to hang one's hat and rest one's head. When I lived in Barrie, I was fortunate that I happened to live in one of the first rings of suburbs... back in 1998, I was only a forty-minute walk from my downtown high school. Thirteen years later, someone living at the fringe of Barrie would be lucky to be able to walk to downtown in twice as much time. Sprawling subdivisions are based on the idea that the automobile brings freedom, but in practice they're practically tools of oppression - if you have to use your car to get anywhere, if you're obligated to fritter away your day behind a steering wheel without any alternative... how free are you, really?
This strategy of building, this insane drive to plow under more and more land, borders on the instinctual now - the industries have been building this way for nearly seventy years, ever since Levittown. We can't allow them to keep on this way forever - their goals are not the same as ours. What we need are livable suburbs, ones built along transit lines like the streetcar suburbs of old, and densification to create walkable communities. We can't let the attitudes of the twentieth century continue to shape the twenty-first, if we want to greet the twenty-second in any semblance of good order.
There needs to be choice - not just the assumption that a car and a house in the suburbs should be the only game in town.