(pseudo)Crystal ball gazing is fun.
I agree, and it has been commented many times across various other online sources.
Their problem is management, rather than products tho.
The product deficiencies are simple, in that the products are now quite old, whereas many products from mirrorless manufacturers are newer.
Nikon's only truly new product at the moment is the D5!!
D810 is D800 re incarnated with the addition of an electronic front curtain, sensor tweak and firmware. That's a 2012 model camera!
D750 is old, D610 is older, Df is older again .. etc, etc.
Customers simply want the new stuff, and Sony give them new stuff almost on a daily schedule! .. so their sales will inevitably be on the up. At what point do the Sony customers feel that the day they buy their current camera, the new model that comes the next month has a feature that they 'needed'!!
(and just to be sure I don't upset the Sony users applecart .. that is said tongue firmly in cheek!)
But since the original A7 in 2014, Sony have released 8 x A7 + the A9 now, ie. iterated every couple of months .. whereas Nikon have had the same model lineup in that FF market(D610, D750, D810 .. with the Df being even older).
So within this next 12 months, Nikon are due to release at least 3 new models, and if they produce a successor to the Df 4 new models.
This is the market where Nikon will be banking on making their profit(they've said so themselves). How well these models are received, only time will tell, and the only true indication of Nikon has a problem, will be in about 3years time if none of these new releases have any impact.
It should also be remembered that Nikon started out making 'mirrorless' cameras, and the DSLR came about as an answer to the limitations of that S series rangefinder. So Nikon don't have problems related to products, and could easily adapt either way.
I don't think getting rid of the Fmount is a truly viable option for them, unless they went with the old S-mount from a million years ago(unlikely)
Back then(in the 50's -60's) they did have Fmount to Smount options so that Fmount lens users could mount the lens to an S series camera. Many lenses were hybrid Smount converted to Fmount designs in the early years.
So in that sense they have experience with both a camera/lens mount paradigm shift, and the experience to do so.
And on the topic of short flange distance issues, back in the film days it made a bit more of a difference to the design of the lenses, so for sure 'mirrorless' camera lenses back in the day were smaller.
But until we get curved sensor designs(who knows when or if that will happen!!) those short flange distance lenses will have a much greater issue with corner shading due to the nature of the direction of the corner photosites on the sensor.
Sony is known to process their raw images with a vignetting routine, which has been referred too many times over the net.
You only see this if you use obscure thirdparty software, where Sony hasn't consulted with the company on how the raw file is manipulated in camera.
So yes .. mirrorless cameras can have smaller wideangle lenses for a given format size .. but in reality only back in the film days. But then again, NO!! .. they generally don't, due to the alignment issues for those corner pixels on the sensor.
Given curved sensors in the future .. this may change for future lens designs.
Take the old(but still current) Nikon 20/2.8 AF-D lens as an example: 69 x 42.5 mm and 270g. Nikon micro motors are known to weigh in at between 50-100g, so with a built in focus motor we'll say 370g.
Granted that this is an old lens, and almost certainly not going to be as sharp as the Zeiss, but I can't understand why the Zeiss is such a (comparatively) lengthy lens at 72mm, when the old Nikon can do with 42mm.
That's a substantial difference, and the major reason must surely be corner shading issues.
As the lens gets too close to the sensor/film plane, the light hitting the corners is reduced.
If you have a magnifying glass, you can easily see this effect by moving the magnifying glass to and fro .. the light circle gets smaller as you move the magnifying glass closet to an imaging surface. Use an overhead light globe and point the mag/glass so that the globe light makes a circle on your desk. Move the mag-glass towards the light/away from desk and the light circle gets larger. Move it closer to the desk and it gets smaller(and focuses more sharply).
You can't deny physics it properties.
For film, due to the nature of the way film captures light, and that there is no orientation of film grain, not a problem, so lenses could be teeny tiny.
But until curved sensors come to market, this will always be a major problem for digital.
Note too tho, that the major problem with curved sensors is that while it all works fine and dandy for wideangle corner shading, the opposite effects occurs as focal lengths get longer. So it's not just a simple matter to deal with.
Curve the sensors for wide angle, and you have a wide angle format camera .. it won't work well for longer focal lenths(where the light rays come in paralell) .. so the corner shading effect then hits the longer focal lengths instead!
So the problem isn't simply solved with a curved sensors .. it just moves the issue .. the answer is really a morphing sensors. One that can curve at the edges for wideangle .. and then straighten up for longer focal lensths. What's the chances of that happeing any time soon.
I think you're going to see larger UWA lenses for mirrorless than you imagined.
I doubt that any new 12mm UWA lens for mirrorless will be all that much smaller than the equivalent 12mm for a longer backfocus DSLR design(for any given aperture range).
Have a look at what the Voightlander 12mm f/5.6 lens does in terms of mad vignetting on a Sony A7!!
And it should be noted that with a maximum f/5.6 aperture that lens should in effect produce no vignetting at all!
I think the appropriate euphemism here is: Swings and Roundabouts!