Right, so I've missed some of the conversation. However the jist seems to be can the US put it up to the 6 nations sides in 10 years time?
Not really. I've said that they could challenge the celtic nations (and, by association, I guess the 6nations). I'd suggest that being a challenge to a team is slightly different from doing them over.
That said, your point about flyhalves is pretty significant. While you may have some positions from other sports at highschool that lend themselves to a fair conversion at a young age (get a fair number of decent LB/RB hybrid players, so tacklers who can run decent lines) theres no history of flyhalves. At all. Time for an American passport to be found somewhere in the southern hemisphere.
As an aside, there's all this talk about the domestic game not being strong enough, without a decent professional outfit, yet Argentina does not have a single professional team, with no hint of this on the horizon. Yes, they have a heritage of the game, but its one that is largely played and supported by the upper classes. Whilst not starting form scratch, they've effectively sidestepped the need for any real domestic facilities.