Regarding Canon. It seems to me that as I read these forums that every time Canon, Nikon, and now Sony come out with a promising body (as opposed to the me too bodies that pop up) we start seeing the doom and gloom speculation on the other competitors.
The thing is, it's usually true.
When Canon came out with a full-frame body, and Nikon said they'd 'never' go full-frame, Canon gained a lot of high-end market share.
When Canon came out with CMOS while Nikon was stuck with CCD, Canon gained market share. Again with the 5D, which made full-frame affordable - Nikon had no reply. Again with the 5D2, with high resolution and video. We're still seeing the market share effects of this early dominance - changing systems is much more onerous than just buying a new camera.
When the D800 came out and Canon couldn't reply, Canon lost a good chunk of their non-action 5D2 users. When the A7r came out, could use Canon lenses and Canon still couldn't reply, they lost even more of their non-action users. It will be the same with the D850 - if Canon can't reply with something with both resolution and speed (rather than one or the other) that's better than the 5D4, they'll be hard-pressed among wildlife photographers (especially) as well as some sports photographers.
The D850, by the released specifications, is going to be a great camera. Notice I didn't say best? Best is a personal preference in many ways. We can all compare DR/ISO charts and scream SEE! but in reality there is so little difference in today's technology that it doesn't make that much difference in real terms. I know there are many that disagree with that but it really doesn't. No one is completely missing a shot due to .70 or so stops of DR. They may wish for better image quality, but missing it? I doubt it.
At the bottom end, the almost 2 stops of DR difference between the D810 and 5Ds certainly make a difference for anyone shooting in difficult lighting (backlit, high-contrast, etc.). There's a reason few photographers outside of the studio (where lighting is controllable) were particularly interested in the 5Ds, despite the resolution advantage over the D810 and A7r2.
If it were only 0.7 stops, it would be far less noticeable.
I see the D850 a little differently I think. To me, with the resolution it has, the great DR it has... it is a direct competitor to Canon's 5Ds (current version). In fact, it steps all over it. The resolution loss of 4mp is not that big a deal when you consider the DR difference between the two at low ISO. In addition you get fps and af that can easily be used for those action opportunities that sometimes arise for landscape/studio/macro shooters. Comparing this body to the 5DIV (which I have) is possible but I don't think realistic. The resolution is too far different, so is the speed and af for that matter (although for my purposes the 5DIV has far more than I need).
Nikon never needed to do any more to compete with the 5Ds. For any non-action photographer outside of a studio with controlled lighting, the D810's 36MP and DR are much better than the 5Ds's 50MP already. Furthermore, it's a 2-year-old body that's due for replacement soon.
Given the similar price point, the D850 absolutely stomps all over the 5D4. For roughly the same price, you can get a 30MP body with 7fps and Canon's second-tier AF system, or a 46MP body capable of 9fps with Nikon's best AF. Both are general-use bodies, with frame rate to shoot some action and more resolution than the speed-focused bodies, and both are around the same price, so it's a valid comparison - anyone who could consider the 5D4 for their work could equally consider the D850, without sacrificing any aspect of performance or paying a significantly higher price. But the D850 does every part so much better - resolution, likely DR, FPS (where it's as fast as some pure action bodies) and AF - that it doesn't even look to be a contest.
To put it another way, the 5D4 didn't have to worry about the D810 - even though the D810 had higher resolution and better base-ISO image quality, it lacked the speed for action photography and was at a half-stop disadvantage at high ISO. The 5D4 remained better for all-round use when action and high-ISO was part of the repertoire. Same with 5D3 vs D800. Not so with the D850 - it does everything the 5D4 does, but better, so is a viable alternative to it.
It's the 5Ds2 it need to worry about. Canon's not going to step back on the resolution - expect 60-70MP - and they've fixed the DR issue in their latest sensors. It's unlikely to be an action camera - I'd expect 5fps or so - but, for the non-action photography role, it's going to look attractive, unless Nikon also releases something with similar resolution.
Does anyone think Canon, or Sony...don't have more technology coming around the corner? We already know Canon were playing around with 120mp foveon sensors years ago, do we think they just dropped it? Sony has pushed sensor technology like no other company currently, do we think they aren't going to make sure they stay in front from a sensor standpoint?
Of course they are, and when either company comes out with the next BANG, everyone will sit around talking doom and gloom about the others. None of the these companies (even Nikon with their shrinkage) is going away any time soon. All of them are making camera bodies that compete quite well with each other in actual use. Each respective "fan camp" screams best according to their chosen brand, but in real world use take a body from any of them and you can get the job done quite well.... certainly better than ever before in history.
It's all about matching capabilities and being able to compete among each segment of users.
Whether you're talking about landscape, studio, commercial, wedding, sports, wildlife, real-estate or almost any other type of photographer or application, you can pretty much categorise their stills camera requirements into three groups - pure speed (where fps and AF are king and ISO 400-12800 the critical area), balanced (where you need as much resolution as possible while maintaining an action-capable frame rate of at least 7fps, preferably 8-10fps, AF is still king, as well as performance from base ISO all the way to 12800 or so) and non-action (where resolution and base ISO performance matter above all else).
A camera from one group can't adequately substitute for a camera in one of the other groups. You can shoot a landscape with a D5, but a D810 will outperform it each time. Vice-versa with sports.
Each manufacturer needs at least one camera in each group in order to compete meaningfully for photographers with requirements that fall within that group. For instance, in the action group, Canon has the 1Dx2, Nikon the D5 and Sony the A9. In the non-action group. Canon has the 5Ds, Nikon the D810 and Sony the A7r2.
But there can be times when, for whatever reason, a manufacturer has fallen so far behind the others in capability that it can no longer meaningfully compete in one of those groups. We've seen that happen with Canon in the non-action category - they were leaders with the 5D2, but, by the time the D800 came around four years later, they were well behind. The release of the D810 and A7r/A7r2 only cemented this, and many non-action photographers abandoned Canon. Similarly, Sony is only starting to become competitive in the speed category (with the launch of the A9), and only in areas where they have adequate lens coverage (it's a whole-system thing, not just a camera body thing).
Obviously, changing systems is a hassle, and a manufacturer may survive one generation of bodies significantly underperforming in one of the categories, but, any longer than that, and people are going to start switching. Just look at the flood of non-action photographers who abandoned Canon, first for the D800, then for the A7r and D810, once it became clear Canon was no longer able to compete in that category.
I'd be interested to see if Canon has an answer in the balanced group (against the D850 and whatever Sony release) or Nikon in the non-action group (against the 5Ds2 and A7r3/A9r/whatever Sony call their high-resolution body). Yes, the D850 is high-resolution by current standards. But it is likely to be coming up against 60-70MP bodies in the next generation. And, for non-action applications, it will be judged by its 46MP, not by the fact that it can also shoot at 9fps and track a cheetah sprinting through long grass.