Look at the Canon 6DMkII. It offers very little over the 6D and even less over competition.
An even better data point is the a7rII vs a7r.
And finally, the main concern of Sony will be to sell as many a9r as possible, so the differentiationn they will want to protect is btw a7rIII and a9r.
Anyway you look at it, it makes more sense to Sony to keep the a7rIII an alrounder camera which means small increases of resolution.
And I believe that the D850 is targetting the same positioning in the Nikon line up, which means a D900 is coming too.
Cheers,
Bernard
But the A7r2 isn't an all-rounder camera. It's basically a 'bare sensor' camera or digital back, with a fantastic sensor and a lens mount that can take just about any lens you'd want to put in front of it, and not much else. Just about the only higher-end camera it beats performance-wise is the A7r, which was even more of a 'bare-sensor' camera - unlike the A7r, the A7r2 can actually AF on a walking person, when using a native Sony E-mount lens.
If you want to shoot anything that moves, the D810 or 5Ds are far better options (depending on whether you needed resolution or DR more) - they're cameras with true all-round capability, albeit a strong focus on resolution and image quality (no doubt the 5Ds successor will use the on-sensor ADC technology seen in the 1Dx2 and 5D4 for greater DR - it's not a budget, entry-level full-frame body like the 6D2). The appeal of the A7r and A7r2 wasn't to those who needed an all-round body - it was to those who didn't care about the other features and just wanted to put the best lenses in front of the best high-resolution sensors. It's no accident that the A7r sold heavily to Canon non-action photographers wanting a better sensor to put in front of their lenses - it was even marketed that way, with a Metabones adaptor being provided free with each camera sold.
The A7r3 needs to be along the same lines - maximal image quality, with everything else being secondary (or not even a consideration). They won't attract the non-action studio/landscape photographers, who were big buyers of the A7r and A7r2, with a body with only minimal IQ upgrades over its predecessor, and minimal IQ advantage over its comparably-priced competition, regardless of what else it includes in terms of AF, frame rate, etc. They also won't upgrade to the A9r, if what they need is a great sensor and not much else. Price is a big factor here - spending $2.5-3k USD is one thing, but double that, when you only need the sensor and none of the other parts, is another thing entirely.
It would make more sense for the A7r3 and A9r to be analogous to the 5D2 and 1Ds3 - equal image quality, but one being a bare-bones system for those who just need the sensor, the other being a high-performance system with top-tier AF, higher frame rate, etc., for those needing it (e.g. photojournalists, or those using it to shoot wildlife/action in addition to landscapes/architecture). Many people would buy one of each - the A9r as their base system, with the A7r3 as backup. Others (non-action photographers currently using the A7r or A7r2) would consider the high-resolution A7r3, whereas they would be less likely to consider a lower-resolution A7r3 or a more-expensive A9r. It'd be a win-win situation.
