I have a couple of comments.
Most people who come to visit do leave. It seems to me that the problems we're identifying are with the visa holders. There are people who visit who don't require a visa. What about them? Are we concerned about Americans or Britons who overstay?
I have a much smaller estimate than one million on status in Canada. I worked on the boat migrant issue back in 1999, and at that time I estimated non-status to be around 50,000 across Canada. That's probably at the low end of the estimate.