AFFORDABILITY ISSUES
You will agree that capping suburbanization leads to higher housing prices? (All empirical evidence does suggest so)
If we cap suburbanization in the GTA and prices go up, can you personally afford a, say $700,000 dwelling? If not are you willing to put up with something less desireable or to move to another, cheaper city?
Many people wouldn't be able to afford that and will either choose to move elsewhere or live more impoverished. Should we see a rise in poverty in exchange for helping the environment? Is poverty or a bad environment worse? (Though one could easily argue that suburbia, if done right, doesn't worsen the environment... I'll leave that point of view for another post)
Firstly, yeah if I had to choose poverty vs large environmental damage, I'd choose the environment.
Secondly, it doesn't need to be poverty. If you run out of room to build single family houses, start making mid rise apartments and high rise buildings, or redeveloping larger lot housing. This will not raise housing prices substantially, and might actually lower prices down than from what they are today (due to current pressure to prevent sprawl.) So, I will not agree that capping suburbanization leads to higher housing prices. Take a look at East Asian cities; density's cheap there. It's also cheap in slums, places with the lowest land values in cities and sometimes without any housing price at all. Under the current suburban model, yes density would cost more. But that model's easy to overcome.
When you have the market focused on single family housing as you do now, any high density is going to be a niche market. This means that high density's high prices become a self-perpetuating system, as only the rich are able to afford high density, while the poorer or less committed people buy up houses that are able to be cheap because of the huge market. When you put pressure on this model by restricting suburban development a bit, as we see already, prices will rise because less houses are able to be built.
But if you've got high density as the norm and detached housing as the niche market for country folk at heart, it's a whole new story. While higher density slowly builds over lower density and infills vacant lots, you have opportunity for people that are fine either way (or even want higher density but can't afford it) to start taking those new opened houses, as well as a large number of newcomers to the city that feel the same way.
These people coming out of the turning niche market suburban housing open up room for newcomers that would really like a detached house, while in total allowing for more people to economically fit into the city without disturbing farmland. Just a comfortable mildly redeveloped GTA could easily fit in 2 or 3x as many people as we have now without intruding on new land, and giving countless opportunities for local business and public transit (also improving the efficiency of public transit by giving density of scale for subway, LRT, and regional rail.)
I'm quite intrigued as to why you'd say that suburbia doesn't worsen the environment.
SUPERIORITY OF URBAN LIFESTYLE?
With all due respect I do live an "urban" lifestyle right now and walking to my neighbourhood McDonalds (or better restaurants) takes longer than driving to local establishments from my parents suburban house, , it doesn't improve my social life (do I need to live within a few blocks of my friends?), and shopping for anything that takes up more than 2 grocery bags is just not possible. Oh and actually driving from my parents suburb downtown is faster than taking the subway from my more centrally located place.
It's kind of hard to note the superiority of driving to subway when our subway network's been neglected for the past 30 years while our roads have been a top priority for the past 60. And even still, are you driving downtown during rush hour? Are you driving downtown from Bowmanville? (which is the distance many would be living in 30 years if the GTA were to continue sprawling outwards.)
In a city built equally for transit and the automobile (eg. London, Paris, New York City,) transit is commonly around the same speed as driving, and the only reason driving is any good is because so much of the population is taking transit. And, transit keeps you active, frees up space for more housing, helps those that can't afford or drive an automobile, and forces you to socialize.
If you need a reason why it'd be good for you to live close to your friends and family, I'd recommend that you go out and get some friends and family. What happens when you're close to your loved ones? You have opportunities to see them more often, especially casually, leading to more enriching relationships. It's not as much of a hassle to go to a "local" restaurant or to invite them over. In the suburbs, your mate from work/school/wherever could be kilometres away halfway across the region, making an intimate (human) relationship hard and further isolating yourself to your household.
For me, it's hard enough getting around the relatively small region of the city I interact with, even in a car. I couldn't imagine someone who's ties stretch from Brampton to Durham or (god forbid) is inhabiting the 2.5x as big GTA that you'd like to see with unchecked suburbanization.