Canon will be supporting the EF mount for quite some time. Their recent statement was that there are no current plans for doing additional development with the EF mount because everything is covered in this very mature mount BUT if there is a significant customer need for something different they will consider a new release since they have that capability.
The only new EF lens I am seriously considering adding is the latest 400 2.8 to take advantage of the weight reduction but I am still using a lot of EF glass I bought back in 2005. Unlike the rapidly changing technology in sensors, A to D conversion, auto focus, etc. which makes it compelling to keep ugrading bodies to keep up with better low light performance and faster focusing, sharp and fast glass that is 20 years old is still sharp fast glass. A 1DX 3 will be the fourth 1 series digital I have purchased but I will still be using almost every lens with it that I bought with my 1D Mark II because I feel no need to upgrade the glass.
I am trying to decide whether to get on the 1DX 3 wait list now or to wait a few months for any early production changes to take place based upon initial run feedback but I have no hesitation about buying another expensive EF mount camera. With current technology, a DSLR is a much better fit for what I do than mirrorless. The viewfinder and feel of the 1DX family is still the perfect platform for sports, if I was shooting landscapes or studio stuff then mirrorless would be much more compelling to me but if that was my area and I was changing lens systems I would also very seriously consider the jump to medium format rather than a mirrorless full frame.
For an all around, mid priced camera I can definitely see the reason for mirrorless. And although not a huge risk, those using mirrorless cameras (or live view all of the time with a DSLR) need to practice the same caution that video cam users have been dealing with for years. The sensor is subject to damage from sunlight, laser, or in some cases even high power ultra bright LED lights (becoming more common as focused beam lights at stadiums) so be careful where you are aiming your mirrorless when it isn't in use. With a DSLR, the sensor is only exposed during actual exposure so it is far less subject to those sensor destroying burns.
And on edit, to add to what Ernie said in an earlier post from August about the pro-market being important: A lot of the features that are developed for the pro bodies trickle down to the prosumer and consumer level offerings over time but would never have happened if the pro-market didn't support the cost of initial resource. The small market is also a good place to "shake out" new technology to further refine it for the mass market. This is a common sort of business strategy across industries, for example GM has done this for decades with the Corvette which debuts a lot of new GM research technology which quickly filters to other vehicle offerings.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
