Oh they will and switching to mirrorless isn't the big deal people think it is. Canon and Nikon will do just fine if they stay the course and let Sony do all the legwork in figuring out how to make a mirrorless camera that professionals want to use.
Sony starts actually stealing market share from Canikon, thus it would make sense that Canikon should and probably will start moving soon.
Its not hard to specify what professionals want to use. I doubt Canikon has any need to let Sony check the waters in this respect. Besides, they neither gain the knowhow nor the production capabilities for mirrorless if they use Sony as a scout this way.
Their only motive not to publish a mirrorless system is not to hurt their DSLR sales. They have a huge stack of investments in DSLRs they want to protect, including an extensive inventory of lenses and lens production capabilities. And all this in a shrinking market.
The two main challenges of mirrorless still havent been satisfyingly universally solved:
(a) Autofocus performance. While the systems are getting better, DSLR autofocus is still ahead of mirrorless. Solutions are the advanced contrast autofocus of Panasonic (currently requires knowing the exact behavior of the lens, but a learning algorithm could be possible) and phase detection pixels, up to the dualpixel technology of Canon.
(b) Sensors that can handle steep angles of light. Panasonic just published a paper that claimed they can manage now 60 degrees with their newest technology. That would be a highly satisfying performance. But when will actual products appear ?
With the mirror box gone, what we have is basically a DSLR in Live View mode. But with the EVF-equipped cameras, we’re not restricted to just the rear screen—we can use the same Live View in the eye-level viewfinder. This is the difference between DSLR and mirrorless cameras, not size.
Well, no.
Theres a *lot* of differences between SLR and MILC. Including *size* - the lack of a mirrorbox allows more compact and higher quality wide angle lenses - assuming that the sensor can handle more steep angles of light.
EVF however - is not one of them. A SLR could have an EVF in lifeview mode. There is absolutely no technical issue stopping anyone adding an EVF to a SLR this way. I have demanded this feature since years and I see other people demand this feature since years. For example I was hoping the Nikon Df would have this feature, given what kind of camera it is.
For starters it would allow using a lot of great lenses that are however manual in a much more comfortable way, allowing both focus peaking and 100% magnification (and, if the sensor has a feature like dualpixel, also digital split screen) for critical focusing. Old Nikon pre-AI/AI/AI-S lenses, Zeiss and Voigtländer lenses (of course also available for Canon), PC-E lenses (or TS-E for Canon) etc could all be used in a very reliable way. With a mechanism to move the sensor forward for the required shorter flange distance, even adapting mirrorless lenses could be possible.
It also would allow much more comfortable video recording. Good video recording with a SLR is another example for something that could be implemented at any time - the camera producers simply choose not to do so.
The only reason why SLRs do not have an EVF is because the camera producers dont want to add one. Just like mirrorless can have an energy-saving OVF - even if Leica M and Fuji X-Pro are the only two cases that actually do so (and only the Leica M has an OVF that DSLR-like doesnt use any current at all).
Give those D5/1D-X and D810/5D-S/5D-III a comparable camera with Live View in the viewfinder, and surround it with the lenses they need.
Sony wont replace the D5/1D-X2 any time soon. Maybe never.
This would be hopelessly futile. Because outside propaganda Sony currently cant offer them the required Autofocus performance as well. The only camera that theoretically can do it in good enough light (apparently up to EV 3) is the A7r II, but from what I heard, this camera doesnt offer enough reliability - the camera very phrone to stall and even worse somewhat phrone to crash.
Replacing the D810 is more likely, especially if no higher autofocus performance is required.
Professionals however are a very small part of the camera market. Thus Sony stands a good chance with richer hobbyists who dont mind the extra hassle from such obstacles.
So whats the advantage of mirrorless if not the size? Frankly thus far there are none that I can think of.
Mirrorless has some strong advantages over SLRs, but SLRs also have some strong advantages over mirrorless. In the very long run, once all the issues of mirrorless are solved, I see mirrorless dominating, but I think SLR will stay, just like rangefinder still stays, maybe better (specifically wildlife and maybe also action shooters might keep using DSLRs).
+ Size definitely is an advantage of mirrorless, only much less than people seem to assume. The major factor of the smaller size of mirrorless are because the sensors are smaller, and in case of Leica, because the lenses dont have to contain autofocus (same for Zeiss Loxia lenses); Sony FE lenses however arent smaller than DSLR lenses per se and often have to be longer. However, mirrorless definitely loses the mirror box and the parts of DSLRs linked to that mirrorbox, namely the pentaprism and other OVF parts including the metering sensor above the mirror box and the AF system below the mirror box. Instead Mirrorless needs space for an EVF and possibly larger batteries, though, though many mirrorless cameras currently instead offer a lot less shots per battery.
+ Mirrorless cameras have a magnitude less parts because the mirrorbox of a modern DSLR is actually a very complicated thing. This makes mirrorless cameras much simpler, more robust, and cheaper. Especially the cheaper part currently cannot be seen in actual cameras because the DSLRs have the advantage in sheer production numbers.
+ If the sensor can handle steep angles of light, wide angle lenses can avoid retrofocus in their optical formula and can thus be made with less elements, smaller, more lightweight, cheaper, and with higher quality.
+ EVF can allow you a view compareable to a medium format OVF even if the sensor is small. They offer perfect 100% framing (one of the reasons why to use lifeview on a DSLR is to get perfectly accurate framing of a shot).
+ If they have an OVF, its rangefinder style ... not really a clear advantage, but an advantage to some types of photography (candid, street).
+ With an EVF, theres a huge number of possibilities, basically only limited by your imagination. Focus Peaking, Magnification, Digital Split Screen, Zebras, Life Histogram, False Colors etc. Of course you can have all that with a DSLR put into lifeview mode, assuming you give the DSLR an EVF during lifeview mode.
+ Since AF is usually contrast or combined contrast with phase, the AF system doesnt need calibration and can be made very reliable.
Issues of mirrorless:
- MILC use a lot more current than DSLR. Specifically they use current all the time. A huge issue for people who want to lie still waiting for their "prey" for hours, namely wildlife shooters.
- Good EVFs are still quite expensive, and the best also require quite a bit of space. EVF currently still have a lot of issues - still some lag, cant handle low light well, too low resolution, flickering with artificial light, and some people with especially poor eyes report they can use OVF, but not EVF.
- AF performance, especially with action in low light, is still clearly inferior to the best DSLRs.