I'm a little conflicted about all of this. I mean, were most of these buildings designed to last? I'd imagine a lot of them would've had to have been knocked down by now anyway.
Old Montreal didn't survive through some sort of preservationist effort. It was really more of an accident. The financial sector moved, and the area went into decay for a good while. I'm guessing there wasn't any development there because no one thought it would be worth it.
Now I'm not saying Toronto is anything like Paris architectually (God I wish), but before the Paris we see now was built, the entire medieval city had to be destroyed. Very little remains from medieval Paris (I think there's one house left somewhere). The point is that even though we've lost a good chunk of our city's historical buildings, we are not alone. It does not prevent us from from building a new beautiful city.
I think I heard somewhere that, after Boston, Toronto has the most preserved Victorian houses in North America. That's certainly something to be proud of. There are a lot of historic buildings we should be proud of. The Royal York, I believe, was once the tallest building in the British Empire. It'll be sad when the condos on the waterfront completely block it from the skyline, but it will still be there. Humber's Lakeshore campus is absolutely beautiful.
You also have to realize that cities very rarely can maintain their historical buildings. If you want to see beautiful 100 year old + architecture, there are amazing areas just outside of the area. Downtown Oakville, for one, has been excellently preserved. Or come out to rural areas. Some villages less than an hour and a half from Toronto have barely changed. To go back to the Paris example, to see medieval France, you go to the towns and villages, not the big cities.
If you look at a city and see only what is lost, you'll never really be able to see what it could be.