Upload & Sell: On
From the standpoint of other "big white" lenses, the only distinguishing 200-400L feature IMO is that it is obviously a zoom.
Now, my 100-400L is also, obviously, a zoom, albeit a slower one, therefore its past usage should be a pretty good indication of my needs for a zoom in that FL range. I guess, in the past 18 months I've probably used 100-400 once or twice, and even that wasn't out of sheer neccessity. Ironically, practically all of those 100-400 shots were done at 400mm. Therefore, the zooming feature was not utilized at all.
The other approach to the zoom need validation is to look at circumstances where I had to back off from 500 or 400mm (primes) because the subject was too close. It was perhaps once or twice in the past couple of years that I wished I had a 200mm lens on me instead of a 400mm or a 500mm one. However, at those times I really wanted 200 f/1.8 or 200 f/2 , and not 200 f/4.
Lastly, I feel a significant 200-400 weight/size detracts from its versatility of application.
For me it would be quite awkward to carry such lens around on my country/city walks just in case its variable FL might become useful. I can still do such exploratory no-specific-mission-preplanned photo forays with 300 f/2.8 IS MkII (+ TCs) in my bag. However, even that represents a sort of a borderline walk-around lens weight-size for me.
Some folks also think of replacing their 400 f/2.8s with a stop slower 200-400L, in recognition of high ISO capable cameras of today. No doubt, from the exposure considerations that could work OK for them. However, there are three other fundamental advantages f/2.8 long lenses have over f/4 ones, which is sometimes not addressed:
(1) Bare f/2.8 lenses enable better camera AF than bare f/4 lenses.
(2) They yield more diffused bokeh/better subject isolation.
(3) They have much better AF responsiveness when coupled with 2xTC. Shooting with f/8 combos is not a happy occasion.
However, we are all different, and so are our photography interests and needs.