We'll know soon enough.
As I said, the point isn't the D5x, it is the exustence of a plan for FX DSLR above the D850.
Cheers,
Bernard
So, D850 vs A7r3 and D900 vs A9r?
Price point would be critical in the relative appeal of the four cameras.
A7r3 vs D850 would be an interesting matchup at the same price point, with the A7r3 likely having a significantly better sensor (maybe 72MP vs 46MP) but the D850 having a faster frame rate and (possibly) better AF (unless the A7r3 is more like the A9 than the A7r2 - it's hard to say, since the A7r2 is more like a prototype than a true part of a mature product lineup).
A9r vs D900/D5x is less certain. How good can they make the A7r3 without encroaching on the A9r? Make it not good enough and it basically becomes a landscape/studio one-trick wonder, as the A7r2 is regarded now, with limited appeal beyond this base compared to the 5Ds2 and D850. Make it too good and the A9r becomes pointless. And where would Nikon source a competitive sensor? Certainly Sony won't sell them theirs if they plan to put it in their own A9r. Perhaps a lesser Sony sensor - 55-60MP?
Certainly, there is room for a top-of-the-line, no-holds-barred high-resolution body. It would be different, rather than necessarily better, than the action bodies - for instance, bandwidth may be similar, for 72MP at 6.7fps instead of 24MP at 20fps - with AF and other features being similar, and a similar price point. Similarly, there is room for an entry-level, high-resolution/DR one-trick wonder, in the same vein as the 5D2, for those who need the sensor and not much else.
In that case, if there is a D5x or D900 in addition to the D850, perhaps the A9r would be the intended competition for both of them, with the A7r3 being an entry-level 'bare sensor' not intended to compete with these bodies at all. Or perhaps the A7 lineup will take a step up (like the 5D line with the 5D3), matching the D850 level of AF and other non-sensor performance, with a third, entry-level A5 line being introduced to compete with the 6D and D600 lines (with the high-resolution 'bare sensor' belonging there). That would certainly introduce symmetry into the lineup of the three major players. But is symmetry really necessary? And can Sony really support - or does it really need - a third line of full-frame mirrorleaa bodiea?
And where does all this leave Canon? The 5Ds2 is a near-certainty. Sensor-wise, it will likely be more A9r/A7r3 than D810 - they've proven their ability to manufacture high-DR sensors, and they're hardly likely to go backwards on resolution - bit the other aspects of performance are far leas certain. Does Canon have room for a 1Dxs in addition to the 5Ds line?
