Are you using the profession with the toughest licensing process to support your argument? When you say that 500K immigrants are needed to replace those doctors/nurses, are you taking in consideration how many of them WILL ACTUALLY become doctors and nurses in Canada? I don't think that many will. For every other profession, I can partially agree; the partially is due to the fact that Canadian market just doesn't pay enough. For many, Canada is a temporary stop before they head down the south for some major salary hike.
Regardless, as I have mentioned before, 500K is not sustainable rate without scaling up everything else. I can see that Canadian government is not scaling up or reworking other parts of the process; just keep pumping those numbers up. That won't end well. Before inviting 500K people: 1) Need enough medical professionals (need to rework the licensing); 2) Need to have affordable housing (ban private equality and massive foreign investments).