I think you'll see increasing numbers of Canadians working for US companies (that's already happening). Of course that's little different than hiring from areas in the US outside the major hubs. Somewhat lower average salaries, but similar skills and work habits, same time zones, etc.
Expanding further, it's already the case that if you're looking for an international employee with similar skills and capabilities to a domestic one, you're going to end up paying similar to what you would domestically. You're rarely going to find someone that you can pay meaningfully less than you'd pay someone in the US (again, outside of places like the Bay area) who has a similar level of skill and ability.
You might find someone with 80% of the ability for 60% of the price, or 50% for 25%, and in some cases companies could save a lot that way. But obviously two 50% developers != one 100% developer in output. And just to reiterate, I'm not saying excellent developers don't exist in other countries; they of course do, but you can't generally expect to get them for a fraction of the price. Maybe a slight discount, which would obviously be worth companies pursuing, but not enough to suddenly put US developers out of work en masse. That's already mostly true, and will become entirely true as remote work becomes more the norm. So it will be somewhat good for developers outside the US, and for US companies; I just don't think the change there will be as massive as it might appear.
I also expect on net it will be great for most domestic US developers as well; there may be added competition, but there will also be more flexibility. The one group that I guess will likely lose out are those who really want to live somewhere like San Francisco, as you'll still have the disadvantage of cost of living, but lose the advantage of privileged access to high-paying companies. (Though as a result, those property values will probably come down over time.)