We're talking about stock parts here, correct? Not accessories? The parts will absolutely be available because they'll soon be selling motorcycles made up of all those bits and pieces. Guy smashes the front end of his transcontinental on a turkey, the outcome isn't "sorry, no parts." Hell, my 2006 Ducati is 1 of 2000 made. A few years ago I had to replace the front fairing...the dozen year old, limited edition paint color, front fairing on a pretty dang rare motorcycle. It arrived in 2 weeks straight from what I imagine to be a warehouse like the one at the end of Raiders of the Lost Ark.
Point is, there'll be parts because they're making bikes. Well, what if BMW makes it difficult for non-transcontinental owners to buy those parts? Hmmmmm. Weird, and I'm not sure how they'd do that....but I also wouldn't put it past them to at least somehow make it difficult. Then you go to eBay and find one that is being parted-out, or you find a transcontinental owner who isn't particularly keen on the knee wind deflector things, or you talk about how much you'd like to see a transcontinental-like thing for the R18 and a guy like Holan or a place like DK or WalzWerk picks up on that and starts making it.