(all other settings to be static) It's a 99.9% sure thing that a lens set to wide open isn't going to produce as good IQ as a lens stopped down one or two clicks!
Fair enough .. it was just a side note and an opinion, but the argument still holds that due to the way viewfinders work, with an EVF image brightness depends more so on maximum aperture set, than do SLRs(in std configurations).
I'm not a fan of std configs, and I prefer a faster(f/2.8 brightness) screen matte. So my fast lenses show bright images. But when I mount slower lenses(eg. f/5.6 or slower) the hit to image brightness is both magnified, and subsequently more obvious.
My pet hate(ie. opinion) was more so for the purpose of highlighting the fact that at f/5.6, the EVF needs to boost it's ISO level to compensate for the two stops of light loss through the aperture(compared to f/2.8). That means grain(my opinion centred around the graininess in these situations).
This is what may cause a gradual evolution of change in the industry(of CaNikon don't step up to the mark). Japan already has 8K capable broadcasting ability(not used much, but a glimpse into the future).
CaNikon are both stuck in the dark ages of HD video recording. Looking into the immediate future, 4K is becoming old hat already(just as it's starting to take off! ) and CaNikon(more so Nikon than Canon) are dragging their feet.
I reckon what will happen in pro terms in many situatons.
They'll stick with tried and tested CaNikon gear for stills, and have access to an A9 plus (say) a 100-400 for video if needed .. or something to similar effect.
ps. clarification of 4K becoming 'old hat' ... while Canon and Nikon both do 4K video, they still stuck in the lower quality modes, and low frame rates. 4K and 60fps should be the minimum feature/ability nowadays. I don't know the Canon's specs, but Nikon's D5 is stuck at 4K and 30fps(and probably a lowly bit rate at that too!).
D5's biggest issue is the recording limit tho.
If the A9 gains a foothold, it'll almost certainly be due to their video capability more than anything else.
Sony 400/2.8 is unlikely in the next 6 months, as they have nothing to base it on .. so it'll be a start from scratch endeavour.
300/2.8 would be a different mater tho. Like Swifty said, focusing mechanism would be their main engineering challenge, and big lens elements need high torque, this also implies high power usage. The A9 is already going to be battery limited due to the EVF, so to multiply this effect with a lens that strains the power system even more is their main challenge.
The question is, if the their 300/2.8 A-mount has the optics base to allow easy transference of an acceptable focusing mechanism for the A9.
400/2.8 if we see one within 2 years will be a miracle (if they haven't started the process yet). 300/2.8 if easily adaptable, in 6 months shouldn't be hard.
Next Olympics(2020) is where we will definitely see how serious Sony is about this A9 market segment .. and how long CaNikon have to up their respective antes to minimise any impact the A9 has on their turf.
Other things to note about the A9 too:
Traditionally, Nikon have a preference to use Sony sensors, so they're more than likely to be in a position soon to use a similar/same sensor .. for example in a D5s. It's the Sensor that's allowing those elevated performance abilities.
So there's no reason to eliminate the possibility of a 20fps capable D5s, with the same video features too in a year or so.
In a sense, Nikon almost has a bit of an advantage over Canon here using this historical perspective as a guide.
Canon need to start from scratch to achieve the same impact. New sensor, and all the other engineering distractions that the top end product entails.