I guess I do see what you mean about erasing those boundaries, however it would be foolish to assume that power and money would redistribute evenly across the continent. Those borders might be gone, but the economic differences and power structures would remain. And given that the US already has more powers, it might be called the NAU, but really it would just end up being a larger USA. During the transitional period between 3 separate countries to a single nation, what structures and policies and such would be maintained? Canada, since the 70's has been willing to move ahead with slightly more left-wing economic policies than the USA or Mexico. Would that be preserved or would Canada's mixed economy and welfare state be completely dismantled to make way for the American free market? Socially Canada has been ahead of the US (even ahead of the blue states) on issues of race, feminism and LGBT rights to name just a few axes of inequality. Would that still be the case, or would the discourse go right back to debates and contention about abortion and gay marriage, debates that were settled in Canada years ago? My concern is that the larger former, American population would be able to completely overrule any former Canadians who might want to preserve or create new versions of services and rights that Canadians enjoyed, but Americans did not.
And anyone who says that all of Canada and the US are culturally homogeneous has clearly never visited Quebec.