It is certainly possible that a larger number of elements would lead to some increase in flare and reduction in contrast. But there you have it, the trade-off of a prime supertelephoto with a smaller number of elements and a 180-400/4 +1.4X TC type lens. I think the value of the zoom is in the flexibility - you cannot just move to change framing with long-distance shots (you would have to do a lot of walking to change framing significantly) and when photographing certain situations it is a benefit to be able to access such a range of focal lengths because the subject may appear at different distances, e.g., in sports photography where you are tied to a fixed position. However, the 70-200/2.8 FL has very high contrast actually and it is a 22/18 design; the 180-400 is 27/19 so the number of groups is almost the same. With the TC in use, there are 35 elements in 24 groups.
High-resolution cameras (such as the D850) may give you some of that flexibility in framing but the zoom can be combined with such a camera to get even more flexibility (and a larger subject in the viewfinder, and more precise / selective AF). If you look at e.g. the Olympics there are many photographers that appear to be jam-packed into a very small space where you might not have enough space to store and change between large lenses. So in such a situation a zoom lens might be the best option, not that I would ever be in such a situation. 
The high cost certainly is going to create some grumbling online.
It seems unanimous that the highly-complex 70-200E FL is a superb lens, besting everything in its class, which is priced at only $2,800.
I think it is a fair bet to believe the highly-complex 180-400E FL TC, priced at $12,300, will be simply spectacular.
It will have its greatest usefulness on the D500, IMO, where its range will be an effective
270mm (TC off, at its widest) to
840mm (TC on, zoomed all the way in).
That is a
~300-900mm professional focal range, without having to carry an extra camera ... an extra lens ... nor even an extra TC.
(It is important to realize this range is on a legitimate pro lens, with Nikon's 'best of everything' on it, not a pro-sumer or budget/entry lens.)
The amount of money you would have to spend on Nikon's best, pro-grade, fixed lenses to duplicate this range would be $5,700 (for a 200mm), $5,500 (for a 300mm) and $11,200 (for 400mm), plus an additional $500 (1.4x TC) to get the 840mm total effective reach at the long-end. That is a total cost of
$22,800,
plus a total weight of
21.6 lb (9.83 kg), not to mention the hassle of swapping lenses/TC, just to get the same ~300 to ~900 effective range using Nikon's best prime lenses + TC.
Viewed in this light, the $12,300 price for the 180-400 +1.4 TC lens seems to be a bargain, and its 7.7 lb (3.5 kg) in weight seems to be a relief—all the while its everything-built-into-one configuration remains utterly hassle-free by comparison.
That's my vote
