I've just been reading Michael Reichmann's overview of the Pentax 645Z, and the following passages from the article struck me as particularly insightful:
Not long ago, nothing could rival MF backs, notwithstanding their usability deficits. That's all changed. The Nikon D800e/810 produces very similar results at a third the price, with a much broader range of high-quality lenses available, also at a lower price. More recently, the Sony A7r puts similar IQ into a camera the size of a cigarette pack. And for the traveller-type, the Fuji X and Olympus Micro 4/3rds systems offer terrific IQ in pint-sized packages.
Remember the accidental 21MP jpeg? An Oly OM-D-E-1M-D-M-E-whatever produces a gorgeous 16MP file. Not all that much of a difference in the vast majority of applications.
Put simply, who really needs a run-'n-gun MF camera? The high ISO is amazing, yes. The file is huge, yes. The usability is on par with 35mm FF, yes. But it costs a lot and is big and heavy. So why? Moreover, any erosion in the discipline required for maximum IQ (tripod, stopped down, etc) will have some impact on the quality derived from the machine. With the quality of the vastly cheaper next-step-down nipping at MF's heals, it's not clear who will find it worthwhile.
What indeed is the point, especially in an age in which the vast majority of images will be viewed on digital screens that - even in the "UltraHD" case - amount to no more than 8MP? On whose behalf would all of the additional expense of medium format digital be accrued, especially in light of the marginal improvements in quality to be had over 35mm DSLRs?
In fact, I'll go further and say that it isn't even clear what point there is nowadays to spending money on 35mm "full frame" cameras, when less than 0.1% of the images they capture will ever be reproduced in formats large enough to accurately convey the advantages of 35mm over good old APS-C - and here I speak from personal experience.
The last time I printed any of my photos was back when I had a D300, and in the years since I haven't once felt the inclination to have any prints made, having neither the storage space to spare nor any real occasion to display physical prints to anyone else. Most of the people who will have seen any of my pictures will have done so on forums like Flickr or Facebook, and viewing downsampled, compressed images on social media sites hardly does justice to what cameras like the D800, D800E and D810 are capable of. What's even more amusing is that these compromised images still look like world-class works of art by comparison with the low-quality smartphone captures raved about on sites like Instagram ...
I think that if most of us who are passionate about photography are honest with ourselves, we will have to admit that given how far sensor technology has come over the last few years, even low-end "consumer" cameras like the Nikon D5300 are now more than sufficient for all but the most demanding applications (e.g. professional sports coverage). The incremental image quality afforded by larger sensors, while real enough, will simply not be noticed by most of the intended audience for typical photographic tasks, and therefore any such expenditure can only be justified on the grounds of one's own personal satisfaction.
I like shooting with a D800 because I can stare all day at the pictures I get from it, even if no one else shares my fascination, and I've been fortunate enough to have the spare income to indulge this personal idiosyncrasy, but I'm not photo-crazy enough to believe that a $10,000 Pentax 645Z would bring me much more satisfaction, no matter how much cash I have burning a hole in my pocket, and unfortunately for Pentax I doubt there are many others out in the world who are. But then again, the very survival of medium-format manufacturers like Hasselblad (and the entry of Leica into this vanishingly small market) is inexplicable except for the existence of sufficiently many well-heeled amateurs* who imagine that spending $40,000 on gear will magically make them better photographers; working pros usually rent such equipment only when they have customers to bill the costs to.
*I.E. your typical middle-aged - and invariably male - bankers, dentists, doctors and lawyers ...