I've seen this conversation on this forum several times and rarely comment, but since I'm pretty bored at work I'll throw a few things out..
1.) I doubt Triumph will ever make a serious dent in the HD market in the states simply because they don't put much effort into it. They have great bikes and an awesome history, but they don't capitalize on it. I can't remember ever having seen a Triumph advert on TV, but I can remember seeing an HD one and a Victory one. Folks in the US, for better or worse, have an attention span that is pretty short, and without some market saturation from Triumph's advertising department I doubt they will ever gain much ground no matter which platform they choose to pursue. Most folks in the US pride themselves on their rugged individualism (even if they are neither rugged or individualistic) which would seem to make Triumph's "Go your own way" theme a perfect approach, but alas, they have chosen not to hit it very hard.
2.) The Rocket is unique. I've ridden a bunch of different bikes and it is, in my experience, one of the most unique experiences out there. However, it is not the be all, end all when it comes to motorcycles which is why I own three. I have an HD bar bar hopper, a Rocket, and a long range cruiser for cross country trips. I know someone will jump up and shout that the Rocket can be a long range cruiser and that is certainly true but there is a Harley shop on every corner throughout the US so no matter where I have trouble on the road I am rarely more than a few miles from a qualified mechanic to fix it and get me back on my way. The last thing I want is to be stuck in the a$$ end of nowhere with a frustrated spouse, a ton of luggage and a wounded ride. Most on here perform their own maintenance and have very little complimentary to say about their local dealership...with a few notable exceptions.
3.) Without a reasonably large market segment of R3 owners there will never be an aftermarket of any appreciable size. Get used to it and start building your own.
OK...boss is pi$$ing and moaning so that's it for now.