I think there's a mix, and it depends of course on each individual person. For some people, wanting to migrate to Canada is definitely part of the reason they apply to Canadian research institutions, whether it's for their research degrees or for their post-doctoral fellowships. I think that immigration incentives would definitely attract researchers.
For me, personally, that wasn't a strong factor in wanting to come to Canada. Returning to Australia was always part of my personal plan. But I can imagine there are lots of people who would find migrating to Canada on a permanent basis to be a strong incentive.