I agree with your last comment, Neil. I think for most hobbiests, there is a real sweetspot/limit around $1,500. They/we will stretch to $1,600 or even $1,700, but not to $2,000 and above.
I just can't imagine why a working pro would buy this lens over the 200-400 f4, except for those few that need a 5:1 zoom for outdoor sports. Maybe this lens is a lot smaller and more portable (I know very little about the 200-400 f4)? And, most of those guys are using the fast primes to get to 300-400mm, since those are also useful under lights an indoors. Their zoom of choice is the 70-200 f2.8.