Leon said:
It does not sound promising for you. If they are considered excessive demand, they will not be allowed to immigrate. You can go to http://www.cic.gc.ca/dmp-md/medical.aspx to find a designated medical practitioner. Make an appointment yourself and discuss with him the odds of your parents being accepted before you decide if it is worth it to apply.
Here is on case where having a private health-care system in parallel with the public one would help these people. Because Canada insists on one universal health-care system, taxpayers must pay for all possibly-expensive health procedures, and therefore Canada might well have to deny the parents passage to Canada. If the parents were able to pay for their own health procedures, in a private system, Canada could let them in without fear of "excessive demand" on the public system.
The main argument against allowing a private system is that doctors would migrate to the higher-paying private system, and leave the public system wanting. But this is easily countered by requiring doctors to work a minimum percentage of time for the public system.
The currently-long waits for public-hospital beds could be reduced if private clinics were allowed to exist.
Yes, it's true that only the rich could afford private health care, but we don't deny the rich the right to buy Mercedes cars, so why begrudge them the right to buy private health care? We commoners would benefit by having better access to the public system (e.g. we could now get hospital beds no longer occupied by rich patients), and (again), immigrants with health problems that would normally bar them from Canada could use the private system and avoid the bar.
It would be a win-win option.
