While the 10 fps performance of the A7R3 might be misleading advertising or have to read the small print - It does uncompressed, full quality, no distortion images at 8 fps. Which still beats Canon offerings.
That is still 1fps better than D850 without battery grip, and 1 fps better than the 5D4 and 3 fps better than the 5DS line.
Canon painted themselves into a corner. How can the 5DSR Mark II possibly be improved to compete with the Sony and Nikon offerings without murdering the 5D4 in the process?
It stands to reason that the 5DSR2 will have many of the same things as the 5D4, such as higher DR, touch screen, and possible DPAF. If it doesn't at least have higher DR - there won't be much point to it at all.
If it goes up to 6 fps, it's a joke of an improvement. If it goes to 7 fps - why bother with a 5D4? If higher, then the 5D4 is completely pointless.
They are also boxed into the AF. Any better AF challenges the 1D series. They can't go backwards to a weaker system. So it will be some kind of unverifably improved 61pt system rehash like the 5D4.
What will they do? As Canon is Canon, they won't update anything for a long time. So long, that by the time a 5DS line is ready according to the Canon 3-5 year cycle, they could do one of two things --
Merge the lines. Instead of offering a separate 5D5 and 5DSR2, just offer a speedy 5D5R or whatever it would be called. 50mp, 8-9 fps, all the bells and whistles except AF, because can't outdo the 1D you know....unless that updates first as is typically scheduled.
Stay the course - and instead make the 5DSR2 an even higher MP camera, 60, 75 or 100mp. Use the massive MP count to justify the weaker ISO and slow FPS. Really push it even more against the medium format world, where 5 fps is a race car. In fairness, the 5DS was marketed to compete in that realm, not against other FF DSLR geared for lowlight, speed and general use. S stands for Studio I've heard.
Then, sometime in 2020 - 5D5 with 8-9 fps and better AF only after the 1DX3 gets something better to keep it on top.
We'll see how this plays out. Canon decided to separate camera traits into different lines. Nikon/Sony used this as a chance to give all-purpose type camera offerings. Canon did well to give a general purpose 5D4 and a high res studio rig in the 5DSR -- BUT....they left out two potential things...a low light specialty camera, and more importantly, a high speed Full Frame that is NOT a flagship. So the offering of specialist camera strategy sounded great at first, until Canon simply left out speed. All they did was give us a mediocre general purpose, and a bare-bones studio camera.
Sony system is immature, still. However, each generation is making very big leaps and each leap is making their system that much more practical to use. AF was always a point of weakness. Well, they have caught up. Won't say better. There's some give and take. But fair to say caught up. If this trajectory continues, they will surpass in AF in all areas by the next generation.
They win on sensor, no issue there.
They are weak on pro features -- ergonomics, settings, layout, lighting control...sure some will argue that, but the Canon is a better machine for getting volume work done.