Toronto isn't in the same league as NYC, London, or Paris. How would a casino "suck the oxygen" out of downtown Toronto? Unless you're suggesting that the opportunities for leisure are so slim that everyone is going to make a beeline for the casino?
It would destroy the surrounding street scape by virtue of its design. If you look at the casinos in Las Vegas, they are all large box like buildings, but also with large space around them and they are designed around the automobile (something Mayor McCheese clearly understands). As a result you get large spaces of nothingness.
If you've even read Jane Jacobs -
The Death and Life of Great American Cities, Jacobs argues (and I agree) that the most sucessful cities are those that are founded on tight city blocks with lots of cross streets. Toronto (by fluke perhaps) is a city built to such a plan; and I would argue that Toronto's inner city core is alive and thriving - downtown Toronto is a sought after place to live and hang out in. (This is why the casinos want to locate in downtown Toronto.)
A large casino will require the demolition of a large tract of already densely occupied land in order to open up just the air-space (just look at the first post in this thread). As a result, you have this huge unoccupied air mass which has been proven to be a city destroyer. (One need only look at Harbourfront and what a cold and uninviting place that is where no-one hangs out.) Where do people "hang out" in Toronto? College Street, the Annex, Greektown, King West, Yorkville, the beaches, Queen West. The one thing that they all have in common is a similar "feel" that invites and attacts people. That feel comes from the way that they have been designed and built - tight. A casino will be the exact opposite and no-one will hang out there and it will destroy the area.
Furthermore, a casino with a huge mall inside of it will serve to take away business from the existing merchants who currently line downtown and give Toronto's downtown its "feel". One need only look at the construction of "the Eaton Centre" and how it destroyed the merchants of Yonge Street to see what will happen.
Above is a picture of the Las Vegas strip. Note the large tracts of nothingness between the casinos and how they are built. This is uninviting space and 0 street scape. How many people do you see on the street? People just drive up and down and drive up and down. (Yes, I've been to Vegas and it's a joke frankly.)
Does that answer your question as to how a casino will suck the oxygen out of downtown?