Total Paparazzi, Law enforcement, and Military, and perhaps some scientific applications. This is where I see the bulk of sales for these lenses. And of course the few (100's? 1000's? ...maybe) well-heeled serious enthusiasts. And some pros - a few. For example, far safer to photograph Kodiak bears as well as Polar bears from a distance: at 1200mm you can double your distance to subject and still fill the frame... ;-)
I will repeat: these prices are a signal of significant price inflation ahead for other lenses. They are priced for the future, and the 400 and 600 will catch up. You will look back on $12k big whites as "the good old days" if you don't buy know.