Mirror-less has been around a over a decade now.... as a way to make a camera, its more convenient to the manufacturer, than it probably is the user. Doing away with the periscope in the lens-mount and porting the sensor display to a back-screen and or electronic peep-hole view-finder, takes away an awful lot of the compromises inherent in the mechanics of an SLR design, while still providing What-You-See-Is-What-You-Get through the lens composition... and possibly doing it slightly better, as the lens mount and mirror wont mask any of the view-finder as it usually does in most SLR systems.
Personally I am NOT convinced that there are an awful lot of 'advantages' to mirror-less for the consumer... its less compromised than a purely optical TTL view-finder; this does mean that lenses can have true focal length, and not be, as most wider angle SLR lenses, 'retro-focus' and have potentially quality reducing elements to compensate for the image plane (sensor) actually being further away from the lens, to make space for the mirror and pentaprism....
As a camera user, this is a rather tentative 'benefit', from my point of view; I do appreciate that same difference, with my old Konica C35 film camera, that has 'true' focal length lens, thanks to no SLR mechanism... but.... even there.... how much of the better optics I might perceive IS actually attributable to that 'better' less compromised lens, and how much due to it being rigidly mounted not interchangeable, and how much 'better' are photos taken with it compared to my 35mm XA2 compact, with its 'equated' 35mm lens, with the lens actually mounted closer to the film plane than the focal length to make it so compact, or compared to my OM10, or any other of my SLR's, with a retro-focus lens to account for the mirror-box.... most of which would be comparing apples with oranges as they generally take more comprised anyway 'zoom' lenses.... probably the closest comparison would be the M42 Pentacon 29mm on my Sigma MK1, as its a very rigid lens mount.... but even there; the main benefit was in 'cost', as much as anything..... and the Sigma and 'budget' Pentacon 29, when new were still as expensive as the made in greater numbers Konica..
The less compromised mechanics, WOULD suggest that the camera maker can possibly make a better lens for the same money... but significantly more likely to make as good a lens for less money, and save more still, on the camera body, not having to make such an intricate pentapsism mechanism, or tolerate having to engineer its grade to a higher level of reliability.....
So, would the camera maker take that opportunity of savings from to not having to make a more expensive pentaprism and mirror mechanism to make a 'better' camera and lens for the same price... or would they make the camera and lens down to the same acceptable quality level, and cream a bit of extra profit, or a bit of both, optimizing sale-ability and profitability? As they historically have done.......
Idea of an alternative to F-Mount... I think that that is where they flopped with the Nikon-One... there was small support for the new lens mount, few lenses and what there was was expensive... brave of them to try... but not the great hit they might have hoped.
The company have sold on the notion that they have maintained their backwards comparability with the F-Mount since 1959, and any F-Mount lens, since then, will fit and work on any F-Mount camera to date..... and that is probably now such a legacy expectation that they HAVE to maintain it, hence the adapter idea.
But How many, even die hard Nikon users have all Nikkor lenses? And in the age of the long-range zoom, how many lenses is that actually likely to be?
Three? Maybe? Short-zoom, long zoom, and maybe a prime, or maybe an UWA or maybe a very long zoom.
Would it impact sales all that much to completely switch mounts? Even if target market is significantly existing f-mount camera users?
Canon have done it. Pentax has done it. Olympus sort of did it.. but none of them had the 'heritage' in the market place that Nikon does....
Ambiguous comment in the propaganda, was suggestion that it was to be an 'Entry-Level, Full-Frame-Sensor' camera.....
Is that an entry level camera to full frame? In which case not many full frame owners, with high end SLR's that likely to come 'down' to a lower grade body? Given the small proportional number of sales of Full-Frame cameras, more likely it's an 'entry' level camera, equivalent to current APS-C offerings... and the large savings of not making an expensive pentaprism mechanism put into offering a much cheaper, but more wow-inducing larger sensor as substitute feature... so more aimed at complete beginners and APS-C users.
How many APS-C users would 'upgrade' to a mirror-less full-frame, described as an 'entry-level' camera, and of them, again, how many would be able to employ 'legacy' lenses they had acquired for an APS-C camera? A lot would have to 'start over' anyway to acquire lenses with an image circle large enough to cover a full-frame sensor....
Personally; I would! The Kit 18-55 that came with my camera wouldn't cover a full-frame sensor, neither would the 55-300. Other two lenses I have, one's 4.5mm fish... would work on full-frame.... but I'd just get a smaller circle in the middle of the frame, and waste even more pixels! UWA I think would work, but I'd loose a lot of the wide I bought it for. In either instance, even where I could use existing lenses, on a mount or not, switch to Full-Frame would beg rationalization of those lenses to best suit at some point.
And IF a real 'entry-level' camera aimed at the same market as current APS-C sensor SLR's; its probably a non-issue. They will most likely never buy anything other than the Kit normal angle zoom that's packaged with the camera; and if they do, odds is they wont buy a Nikkor lens anyway, but a 3rd party one.
The F-Mount 'legacy' would be just that, and the national backwards compatibility, unlikely employed by very many... I mean, how many use Pre AI Nikkor lenses on an AF Digital camera? Value there is the idea you might..... even if actually, you probably cant, or the lens would have to be modified or adapted to do so!
The notion then suggests that Nikon have finally recognized that the F-Mount and the mechanical SLR probably have had their day, and that mirror-less is likely the only way to go in order to keep the costs, of particularly consumer grade cameras, competitive.
The Fifty-Plus year old F-Mount? PROBABLY has long had its day; the acclaimed forwards-backwards comparability HASN'T really been there for at least a decade if not more; The legend however remains, and without the need for a large mirror housing, a 'cheap' adapter to push an old lens forwards on the mount to mimic one, is then a fairly cheap way to keep that legend alive, and meet a customer expectation, that's probably not all that relevant.
Ultimate question is cost vs performance......
And just how good may be the camera and its lenses, compared to existing offerings...... and that full-frame sensor, and high mega-pixie count is likely to generate a lot bigger acclaim than its effect on image quality probably warrants.. question will ultimately come down to is it worth the money.... and both questions remain in speculation.....
But other than allowing expected TTL composition, and saving cost/complexity of a mechanical mirror and pentaprism... how much may be filtered down to the end user as 'added value', is just as much in question.
Mirror-less, architecture, alone, doesn't grantee anything, other than an alternative architecture!