There are already a ton of cameras out there that don’t use mirrors. For example Fuji, Panasonic, Samsung, Olympus, and the Sony f models. The technology to be mirrorless has been here for years. In our phones, in video cameras and many compact cameras like the GoPro. It’s nothing new. Sure there is a difference between a video camera and a still camera but not really. The cameras of the future will probably shoot a stream of video where you can choose the one photo we want out of that stream. Nikon already has the V1in it's mirrorless lineup. The most important thing to take away from this is some of the cameras listed still can take Nikon or EOS mounted lenses with the required adapters.
The big disadvantage of a mirrorless camera for a professional is the true image no longer reflects into the eye piece but will now be rendered to a LCD screen. Which means the photographer will be unable to see the true colors and saturation of the scene through the camera's lens. This also creates other problems. If it's very sunny out it's going to be hard to see your LCD panel. Possibly a deal breaker for some pros.
So should we be in a panic about not buying new lenses and gear because they will be obsolete when or if Nikon and Canon introduce a professional line up of mirrorless bodies? I don't think so. It’s highly unlikely they will just abandon the professionals who have invested tens of thousands of dollars in their equipment. Especially when Nikon and Canon are at such a close arms race. Sigma, who also makes great lenses and camera bodies even recently said it will never make a mirrorless camera. It's a foggy crystal ball.
My bet is professionals will always want a bulkier, heavier weather sealed camera body that feels solid in the hand. With a bigger camera you can include longer lasting batteries, better cooling systems, better auto focus systems and a more balanced body with big lenses. While the consumers might like the more nimble smaller and lighter weight design for a less intensive application.
Professionals will also always need a viewing system that allows them to still frame up shots when doing night photography or heavy ND filter shooting. This is more difficult on a mirrorless with current ISO ratings because the image is shown on an LCD screen where as with a standard DSLR you're seeing the image the way your eye sees it.
So it’s likely we will not see a drastic change in design from what the professional DSLRs are today. But we may just see them start functioning without their mirrors. But solid big cameras are just awesome. Like driving a big pickup truck or SUV. I feel so much better holding a Nikon D800 than a Nikon V1.
So my final thoughts for DSLR vs mirrorless is we will probably see a split in products. The pro model DSLRs that have some weight, good grips that may or may not need mirrors. And a smaller compact mirrorless design that lack ergonomics, long battery life and convenient I/O. I hope the DSLR is around for a long time, since I would really miss the sound of the DSLR when I press that shutter button. Until next time....