There are a plethora of…

There are a plethora of reasons Greenbelt development is an objectively bad idea. It absolutely should not be conducted.

The many wetlands within the greenbelt are critical for natural water management within the province. Developing on them means expensive, perpetually ongoing maintenance if there isn't complete redirection and displacement of that water. Additionally, if the wetlands can be drained and seasonal incoming water redirected away, where is all that water going to go? It'll either flood somewhere else, or require new infrastructure that requires maintenance.

There will of course be impact on the wildlife, which is less immediately obvious. Biodiversity is critical to ensure healthy ecosystems. This will end up having negative long term effects, including but not limited to worse air quality and worse climate change (greenbelt trees felled, which provide oxygen and remove carbon dioxide from the air), worse crop yields (encouraging pollinator decline), and mental health (the Greenbelt is the only greenspace many folks in the GTA have access to).

The Greenbelt also has a cooling effect over the GTA. Thanks to the river valleys, a lot of heat is able to be trapped. With summers only going to get warmer, and energy generation an increasing concern, any way we can reduce usage is a win.

The Greenbelt is also some of the only space for wildlife in the GTA. See above for some benefits of biodiversity. Ecosystems are delicate and easily put out of balance, even if the damage isn't immediately visible. A deadzone at the heart of the province isn't a good thing for anyone.

Additionally, generally urban sprawl that North America is accustomed to is not sustainable. We need to focus on building up, not out. Densification (not even just skyscraper apartment towers, but town houses and low-rise buildings included) has the benefits of reducing citizen's need for cars, by making everything close by. Walkable cities encourage non-car travel, which in turn reduces carbon dioxide emitted in the province (reducing air pollution), and increases citizen health (walking has incredible health benefits for how little effort it requires, and having towns that encourage walking and cycling will indirectly promote healthy lifestyles).

Financially as well, cities that build up not out spend less money on infrastructure. Guelph is the perfect example of this, as they took on a policy of building up the core rather than sprawling out and as such are profitable, paying down old debt with capacity left over during trying times or for large projects. Financially solvent cities are those that learned to not over extend. More businesses in the same space as before means more businesses bringing in tax money, and less infrastructure to spend money on maintaining.

To conclude, Greenbelt expansion brings very little to the table, and is at best a short sighted plan. Keeping to areas that are outside of the Greenbelt or already developed, and building up instead of out, stands to pay for itself in the long term financially and ecologically.