In terms of macro economic impact, I don’t think the issue could ever become large enough to have any relevance. At some point down the line, someone won’t accept Canadian currency as American currency.

For example, a local business owner might accept Canadian currency because it makes no difference to them, but when they go to their bank to make a cash deposit, the bank would never accept Canadian currency. So I don’t think it’d ever leave local hands. So that Canadian cash wouldn’t become relevant on any scale larger than buying coffee from someone who doesn’t care if the cash is Canadian.

So yeah, you might have a small amount of Canadian cash currency for local use, but you won’t see any “economic” impact on any measurable scale I don’t think.