If I were going mirrorless, I would be looking at all of the offerings across the field. When I upgraded from a 1D Mark II to 1DX and 1DX Mark II bodies, Canon offered exactly what I was looking for and it also allowed me to continue using my current EF lenses.
I agree that mirrorless is the future but the future is still the future and for me there is no way I would take any of the current mirrorless offerings over the 1DX and 1DX 2 bodies. I shoot a lot of field sports and the slight lag using what is effectively live view all of the time for mirrorless would drive me nuts. I would also want to do a lot more research into the impact on image noise in low light settings with a sensor that is constantly active and thus generating increased thermal noise. The lowered weight benefit is of little benefit for most of my shooting because the heavier body provides great balance to long fast prime telephoto lenses and the increased power budget/reduced battery life of a mirrorless makes a second battery mandatory for many uses offsetting some of the weight advantage.
The 1 series bodies equipped with a weather sealed lens (most but not all of the L series, for example) won't have a problem with light rain. I have Canon rain coats for my 1DX and 1DX II bodies that fit the 300, 400, and 70-200 2.8 lenses and body because these are setups I do use in the rain and if there is a chance of moderate to heavy rain they are in place but they are somewhat awkward which is the price of keeping the camera dry. But for possible light to moderate rain with a 1 series body and weather sealed lens, a good camera bag will work. I use a Pelican hard case for transport and it provides perfect waterproof storage when the gear is stored at an event but for walk around with something smaller than a L series fast telephoto prime (like the 24-70 and 70-200 2.8 lenses) I have a couple of Lowepro AW series shoulder bags that are of a weather resistant fabric and have their own self contained raincoat that emerges from a compartment to cover the bag.
There is a WiFi accessory that connects to the 1 series bodies but it is sort of awkward and it isn't really something you would want for general use in most situations. I use the RJ45 ethernet port to rapidly transfer images to my computer for later processing. It is fast and I think RJ45 connectors are far more rugged than using the USB connector or removing the CF cards with their somewhat fragile connector because either is more likely to fail in the sort of high duty cycle usage my cameras see.
In around 5 years I will probably be ready to upgrade to new cameras and at that point mirrorless is a possibility depending upon where the technology is at that point. Because I would also need to switch to a new set of lenses at that point, I would look at the offerings from Canon AND the competition. But at this point, if I felt my 1DX series bodies were worn out I would not be switching to the mirrorless system from any of the manufacturers based upon my shooting preferences and needs, others will have different photography preferences and needs.
For limited 4K shooting, any current camera body offering that feature will suffice but as Waddizzle noted they all have limitations starting with 4K is an added feature and not the prime design motivation for these bodies. To escape the EU taxation on video cameras, all of the current DSLR and mirrorless offerings will have the sub 30 minute time limit. And like the mirrorless camera in its native live view mode, you have to be a bit cautious not to expose the camera in video mode to something that will damage the sensor such as intense sunlight through the lens. My daughter needed some sports video and after shooting a little with my 1DX II I decided to pick up a XF-400 camcorder which was much better suited to that task. For high end 4K video, then a Cinema EOS body is the Canon choice but I didn't need that sort of video versatility and performance.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
