99% of my colleagues and friends including me work for US-based clients so if they move to Canada on PR and eventually get citizenship, I am sure most of them will move to the US because getting a high-paying job there for them will be very easy.If we're being honest, one of the issues is the turnover rate for immigrants. Many of them end up leaving for greener pastures elsewhere.
So in the end, you have to replace them as well.
I know people whose entire social group has disappeared because most of them left for other countries.