Technically, a single currency would make sense. However,
A) You KNOW the Americans would insist on the 'American' Dollar being the defacto standard. Too bad it's not as pretty as ours. Even Americans love our multi-coloured currency.
B) You couldn't then have their dollar drop as our dollar rises. One currency, and they'd drag us down with them.
C) The CRTC crows about our losing national identity because of US TV. What is the effect of losing our own currency, do you think?
D) Look at the grand plan they had for the Euro in Europe. It SHOULD have replaced everything and became the defacto standard. Instead, the seperate countries insist on keeping their pounds, francs, deutchemarks, kronen, turtle shells, etc...etc... Net benefit for the expense? Near zero. Sort of like our gun registry.
And don't be so pleased about the US' collapse. You know that when things get REALLY desperate for them, they'll start invading north and south for new lands to pillage.