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  There used to be surface parking lots all over Toronto, and a ton of those are now giant apartment/condo buildings. I absolutely agree that was a "better use of land" but it also drives up the cost of living as the city grows. 
  There are small-medium sized town examples where maybe it's a townhouse development or low-mid rise that goes in where there previously was a factory or mall, and where single family homes used to be there are zoning changes happening. A lot of that is adding people to existing land used for housing in ways that I can get behind, and I agree that form of "build up, not out" does seem like a great housing solution.
  Until you see the giant big-box stores that buy huge chunks of the land outside of the town you just "saved from houses" to service the significantly expanded population in that town because the increased density downtown made large property with big parking lots more expensive, because most towns with 1000 - 20 000 people can't afford to put in underground parking or vertical garages, but they can build a bunch more surface lots outside of town. 
  If towns and cities in the Greenbelt are going to continue to grow because affordability is driving people away from the GTA/other urban centres, they are going to have to build housing, roads, schools, hospitals, nursing homes, grocery stores, and the things people need in their environment to live there. I don't see how they can build all of that "up not out" without impacting the surrounding environment, AKA the Greenbelt.
context: Well, the alternative is buying existing housing and tearing it down to replace it with higher capacity housing. This form of "build up, not out" doesn't require the use of more land.