Canada is only ~10% of the U.S. population. I'm not sure in terms of developer numbers, but if percentages are comparable, that's a pretty small effect on the overall market.
I wouldn't consider increasing the labor pool by 10% a small effect.
Regardless, and once a company has figured out hiring remote for Canada, then why not add Mexico which through NAFTA/USMCA has sorted out IP type issues. Then once MX has worked well why stop there and repeat across South America. So, the reality is likely increasing the labor pool size every year for the next decade. Those annual small changes compound and lead to dramatic changes over the long term.
Take a look at what happened to manufacturing jobs in the US over the past few decades. Remote work has the potential to transform things just as much.