The article is correct ... there is a lot of land available for development before you get to the greenbelt.  It's known as the white belt.  Land available for development that won't be serviced until new density requirements are met within city limits.
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The issue to discuss is how much density is appropriate and at what rate should density be increased.
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There will not be enough land for all that want a detached home ..... the problem will arise now or down the road ... better to face it now and keep the greenspace.  That can't be replaced.  The impact can be mitigated with corresponding improvements to public transit to keep people moving in the more dense urban areas.