Here's the location in question:
http://maps.google.com/maps?q=43.851...10092&t=h&z=16
It's right beside a Go Train station, which would be good for them.
Toronto is the fourth most populous metro area in North America behind New York, Los Angeles, and Chicago. Each of those cities have more than one 15,000+ capacity arenas in the area.
The Allstate Arena in Chicago seems to do okay for itself even though its biggest permanent tenant is the AHL's Wolves. So, even without an NHL team, the arena has the potential to do okay for itself as just an entertainment and concert venue. Of course, I'm sure it will have 150 private boxes that they'll want to be able to rent out year-round, and they'll need a big tenant to do that.
If I were a resident of Markham, I would definitely be concerned about facing the same problems that Glendale is having now. Almost 9 million people live the "Golden Horseshoe" but Markham is on the edge of the high population area.