If most American startups aren't able to hire American workers at competitive pay and start moving little by little to (say) Canada, aren't there a risk that the whole VC/startup ecosystem will slowly move there? If the Apple/Google/Amazon of tomorrow are Canadian, how does that serve USA or American workers?

It's a short term vs long term balance. As an American you should want US to remain dominant in the startup ecosystem. Restricting startup access to skilled workers is counter-productive in the long run, even if it serves your short-term interests.