I would prefer it if this doesn't turn to be another MF to smaller format comparison for image quality or the ability of one to see the difference. Undeniably, there where thousands of discussions about that in the past, but where rather comparing oranges with apples as the technology used in one format was CCD and on the other Cmos...
I believe there is nobody that can argue that a 70% bigger sensor, has significant (and visible) advantage over a smaller sensor of similar technology for all aspects of photography.
Despite the differences in quality (or not) between Cmos and CCD sensors in the past, there where many "forced" (in a way) to use an MFDB because they had to use technical cameras alongside their DSLR equipment, or because of the high sync speeds that leaf shutter lenses would offer them, or because they had to use "true color" multishot technology as to achieve what couldn't be otherwise be performed up to the required quality. It did came at a price, but if it couldn't be done otherwise... then it couldn't!
Then Cmos sensors came (with a huge delay) and vanished all the disadvantages one could ever claim over smaller formats, but the size and the price. Now the GFX comes along to terminate both the size and the price disadvantage, only asking a small amount on top of DSLRs, which I believe, many will consider "sensible" for the (undeniable due to sensor size) quality increase.
OTOH, the minimized mounting distance makes the camera almost as usable as an MFDB mounted on a sliding back when used on a view camera and usability with leaf shutter lenses is feasible which retains the advantages of MF over Dslrs for particular uses, leaving only multishot (for the moment) out of the equation and then letting out the mirrorbox, of which the presence with MF has less uses than with smaller formats and gives an advantage only for certain kinds of MF applications.
Therefore, it is logical to conclude that there will be an attraction out of the existing market of high resolution DSLRs, other than "stealing" customers to some (unknown) extend from the considerably pricier modular offerings that already exist in the market.
The question then is how many, or what percentage, out of the high resolution DSLR users can be attracted... IMO, the amount can be considerable (up to many tens of thousands).
To start with there are the "wedding" pros, which using MF adds them prestige to their business, then many of them use fast MF lenses with film and DSLRs for lower lighting conditions and adding this camera will allow them to share their lenses of the MF platform they use and abandon their DSLRs altogether (shrinking the size of equipment to be used is always among the first priorities a wedding pro has).
Then, there is the hundreds of thousands of (unsupported) platforms from past legends (Bronica, Rollei and many more) that where left "orphan" from entering the digital era and that their lenses can be (finally) come alive once more.
Next, there is those that print big and simply couldn't reach up to a MF system.
Then there is the landscapers that use cameras with movements with film, where this camera can offer them much more flexibility by only adding a T/S adapter on it.
Next, there is architectural photography pros, that (again) through a well design adapter, can turn every wide image circle lens to a T/S one, or use the camera on the rear standard of their bellows camera.
Other than the above, there are the passionate "enthusiasts" that are in an "upgrade" path anyway, many of which will find "jumping to MF" a natural process, either if they need it or not.
And then, there is always many that believe their skills are advanced and what their photographs luck, is better equipment.
Now given that there are hundreds of thousands (maybe even a million) of high resolution DSLRs (both "mirrorbox" and "mirroless") sold each year, Fuji's decision to price the camera on what would be the "natural top" of them, seems a very wise decision as it may attract even wealthy starters. OTOH, the camera did "shadow" everything else presented in Photokina and this is a hint on the market appreciation that it may have.
Given all the above, but the coming of the Hasselblad X1D on top of that as to enhance GFX's presence by adding a nice product on top and "change the world as we know it" ("stolen" from the REM song), I do expect that many will be surprised with the numbers of marketing share that MF may achieve... I think that Fuji (and Sony) will consider anything else than 25K units annually a failure and I won't be surprised if the total MF market will approach or exceed 50K units.
EDIT: Additionally, if a "FF" 54x40mm size sensor will be added by Fuji (or other mass maker) in the (near) future, it will provide a natural (further) upgrade path, which can lead of MF regaining its "past glory" (and numbers) as it was with film...
