https://www.dpreview.com/news/3426999280/sony-a7r-iii-promises-faster-bursts-better-focusing-and-longer-battery-life
No increase of resolution contrary to the rumors spread by some here.
Cheers,
Bernard
There'll be a high-resolution version. It just isn't clear what the name will be (and it never has been). The A9 could just as easily have been released as the A7s3 - after all, the A7r3 also has the dual slots, battery and AF system of the A9.
The A7r3 is, quite clearly, a direct competitor to the D850. Similar (slightly less) resolution, and similar (slightly faster) frame rate. No focus stacking, but it has pixel shift. It's no longer a resolution-focused, non-action camera, but a general-purpose, action-capable body which leaves room at one end for the speed-focused, low-resolution A9, and at the other end for a high-resolution, slow-shooting body.
It will live or die by its AF performance. The coverage is certainly good (68% coverage by area means PDAF points reach 82.5% of the way to the edge on average, which covers all the points you'd actually want to focus on) and accuracy is almost guaranteed, but the main question is speed and tracking ability.
If it focuses and tracks well (matching the D850, and in the same ballpark as the A9/1Dx/D5), it would be a great wildlife and field sports camera, particularly combined with upcoming superteles (maybe that's why the 400 f/2.8 was announced at the same time). If it doesn't, it'll just be another camera theoretically capable of a fast frame rate, but with no good way to use it.
A fast-focusing A7r3 would make a great companion to whatever high-resolution body Sony decides to release.
I'd be much more interested in a 500 f/4 or 200-400 f/4 than the 400 f/2.8, but, if Sony can design one and make it focus well (and 400 f/2.8 is a classic for field sports), then you can just as easily do it with the other two.
Where is Canon in all of this? Performance-wise, the 5D4 is a pygmy compared to the D850, and likely to this one too.