With Film, image quality depended mostly on your lens, film choice and expertise. The camera merely was a tool that brought functionality, reliability and durability to the table. With digital bodies, image quality has always been limited by the camera (sensor sub systems, DSP, electronics, etc.).
With the 36MP D810, we have already hit the plateau in terms of raw megapixels. A significant portion of DSLR photographers deem 36MP too much, at least compared to trade-offs in other attributes like FPS, total frames buffered, file size storage and bandwidth issues. Hence, they have determined that image quality attributes gained by more pixels already are good enough at a lesser figure than what is currently available. Now the 4.9μm pixels size of the D810 is still well above that of the DX D7100 at 3.9μm. A full Frame sensor with 3.9μm pixels would be approximately 56MP! While obviously, technically feasible, I doubt from a practical standpoint we would ever see such a sensor, or if it would actually yield better image quality. I suspect the D5 (Edited) will have 24MPs or less because of this, though there may be a higher resolution variant released.
There are, of course, other attributes to image quality beyond resolution as proxied by MPs. Dynamic range and higher ISO performance being two biggies. Any body have ideas as to which attribute might be the next to plateau?
The Point
Not sure this has a whole lot of significance to a professional or the wealthy as they will purchase incremental upgrades to quality based on value or status. But for the amateur, somewhere in the plateau is where investment in new camera bodies make sense. The plateau can be caused by technical or practical limitations.
Prior to digital I shot a Nikon N90/N90s (forever) while playing with the Coolpix 9x0 series (900, 950, 990). I started in DSLRs with the DX D300 and currently shoot the D7100. I determined that I would jump back to FF only when the technology progressed (started to plateau) to the point that I wouldn't mind getting stuck with that level of image quality for an extended period of time. (Example: I have a 18 month old Samsung Galaxy Note II. The Note 4 is better in lots of ways, but I don't really mind using the Note 2. I'd hate to be stuck with a DroidX!) I personally think the D810 represents a seminal mark in DSLR evolution. In fact, other than resale value, If I was offered a free camera, I'd rather have a D810 (or to lesser extent due to 24MP and idiot dial interface, the D750) than any other current Nikon or Canon body! The D810 is that camera with which I wouldn't mind getting stuck! Glad I resisted biting on the D600, D610, D800/800e. While there will be better DSLRs down the road, I suspect they will be only small incremental advances to what the D810 offers. (Not that I won't lust after them!)
The larger point is that once the DSLR platform stabilizes, similar to how PCs have, the manufacturers can go back to doing what they should be doing, making them function better and consumers can be confident that their purchase isn't going to be 'outdated' in a few years time. I like that because I hate switching cameras. Once I spend the time an effort to learn a camera, I much prefer making images than spending time relearning a replacement tool.