These lenses sell to two markets:
1) Large photo agencies that supply them to their pros.
2) Affluent amateurs...doctors, lawyers, dentists, executives, business owners, etc...
I think the actual amount of professional photographers buying these would be very low. Sports pros get them supplied for the most part and wildlife photographers that can actually make a living are a rare breed.
I agree, and my thought would be area 2 being the larger Market.
Good friend of mine is a Pro Photographer & shoots the D3x + Nikon 200-400f/4 V1, he won Wildlife Photographer of the Year 2013, he simply can't afford the new Canon Big Whites (but would move to the White side if he could), and Nikon South Africa doesn't support Pro Photographers with Loaners, it's Sad but True, not too many Pro Photographers are making a success of a Business surrounding being a Pro Photographer, mostly (at least in Africa) make a living around the Safari Business, Photography being an adjunct to that Business.