It is not clearly supposed to fail just because it isn't released in the US.
If Nokia makes money on it, it clearly is not a failure, and probably will have a follow-up or two. In the long run WP will be the main smartphone OS for Nokia, but this doesn't mean that Harmattan is dead. It will continue as a niche OS as stated by Elop.
As I see it americans are in no way ready to put their money where their mouths are (quite literally), and this prevents them from getting the N9, and lots of similar devices.