EF lenses and the Mirrorless future

Messier Object wrote:

While I'm confident that there will be a 1Dx Mk III, 5D Mk V, 7D Mk III and 90D etc, it's almost inevitable that someday Canon will announce an end to their DSLR lines.

Where will that leave EF lens owners ? Will we be expected to abandon our EF lenses and buy into a whole new generation of mirrorless lenses ? Will Canon offer an EF to Mirrorless adapter to help us through the transition period ? Will the adapted EF lenses perform as well as a new breed of Mirrorless lenses, or will they be a hobbled legacy ? Or will Canon design the new Mirrorless bodies with native EF lens mounts providing full performance equal to or even better than the last DSLR bodies ?

As an EF lens owner, what would be your reaction . . .

Peter

EOSM + EF 100-400mm f/4.5-5.6L IS USM II lens + EF 1.4x III Extender. .
I use mirrorless Canon EOSM cameras and Canon DSLRs.
Sometimes I use them side-by-side...  Especially if I want a wide lens with a second cameras that uses a strong zoom.  I didn't even bother buying an EF-M lens (native EOS M lens) until almost 2 years after buying the EOSM camera.  Whilst I'll concede that the majority of EF-M lenses are much smaller and lighter than EF lenses, the image quality they produce is very, very good. 
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I'd also say you're right about Canon eventually making a mirrorless camera with an EF mount.  At the moment, the EF lenses can already mount to an EF-M lens mount via an adapter. 
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Canon released their original EOS M camera body with an EF-to-EF-M lens mount that enabled users to mount EF and EF-S lenses to the EOS M EF-M lens mount ring.  I already owned DSLR cameras and quite a few good L-series EF lenses when the EOSM was announced, so I wasn't interested in an EF-M lens until I saw the results from the tiny but optically excellent EF-M 22mm f.2 STM lens... and then the sharp and wide EF-M 11-22mm f/4-5.6 IS STM lens.  These lenses make the EOS M series a much lighter and easier to use system to carry when larger and heavier options from DSLRs are not appropriate.  Some venues treat all DSLRs as "pro cameras" and some aircraft have luggage weight restrictions.  There have also been times when I know a larger DSLR with larger lenses would draw the wrong type of attention... and I overheard someone just three days ago remarking that my large lens was of considerable interest to him as I walked past him ...as he was making a drug deal with another person.  I was happy to make it back to my car.  If the image quality is matched, then surely the smaller and lighter option is the better one to take with you.
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I think that the EF lenses generally work very well on the mirrorless cameras with few exceptions.  I spent the day before yesterday on a beach with the EF 100-400mmL f/4.6-5.6 IS USM II lens mounted on my EOS M6 mirrorless... and the new DPAF APS-C sensor allows for fast and accurate AF.  I was able to read the text on a small sign across a body of water in summer heat that was more than half a kilometer away (576.3 meters).  For those who already own EF lenses and have invested heavily in Canon glass, they will be ready to switch to mirrorless (if they haven't already).  If their DSLR's fail or they need to upgrade, there's already an opportunity for them to swap bodies and know that their lenses will continue to perform.
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I do like the effects from several of the EF lenses on the APS-C mirrorless cameras.  The EF 24mm f/1.4L USM II lens is a prime example of a lens that performs admirably on the mirrorless camera bodies. It produces much stronger bokeh than the similar focal length on the EF-M 22mm f/2 STM lens... And the EF 100mm f/2.8L Macro IS USM lens works extremely well with a stronger focal length equivalent than on a Full Frame DSLR.  The Bokeh from the EF 135mm f/2L USM lens is exceptional on the mirrorless cameras and I'll often use this lens for portraits or wildlife.  Oddly enough, the 135mm lens produces amazing results on both FF and APS-C mirrorless cameras.  It makes sense though since the APS-C mirrorless are virtually just APS-C DSLRs.
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APS-C compared to Full Frame - with telephoto lens and 2x III extender.

EOS M6 + EF 24mm f/1.4L USM II lens - an incredible combination
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Benefits of APS-C with EF lenses....
Surprisingly, there's some benefits using APS-C sensors with EF lenses.  The use of the Canon Mirrorless cameras (which are presently all APS-C) means that the crop factor associated with their APS-C sensor increases the perceived focal length by an additional 1.61x magnification as a result of the increased distance from the rearmost lens elements and the sensor on the camera.  Whilst 'cropping' the images by 1.61x, the result is an increase in subject size.  So Macro lenses allow you to get closer to the subject by increasing magnification. 
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And Telephoto lenses give you a little more zoom.  On the APS-C sensor (eg the EOS M6 mirrorless camera), the EF 100-400mmL II lens and an EF 1.4x III Extender gives me an incredible 901.6mm (equiv) focal length with full Auto Focus.  A 2x III Extender on the same lens will give me an astounding 1288mm (equiv).  As a result, I tend to use this combination for shooting wildlife and the moon.  I have also found a way to attach the two Series III extenders to the lens using an EF-12 Tele-extender tube... which then gives me a manual focus lens with 1,803.2mm (equiv).   When you're using a lens with plenty of reach, you normally want as much zoom as you can possibly muster.  So being able to crank out these obscenely long focal lengths using an APS-C mirrorless simply adds to their usefulness.
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FF Vs APS-C
The main reason to buy Full Frame is the better signal-to-noise ratio with higher ISO settings.  Generally you're forced to shoot with slightly higher ISO settings when shooting in low light with an APS-C camera.  So with a FF camera you can shoot with faster shutter speeds and lower ISO settings than APS-C cameras allow.  Eventually this may change.  So there are still times when I will reach for my DSLR cameras with Full Frame sensors.  Usually when I need to shoot in low light.  With the inclusion of the new DPAF sensors in both DSLRS and EOSM cameras, focus speeds are no longer the issue they once were. For example, using a 400mm focal length lens on the original EOSM mirrorless camera was slow and tedious.  The same lens, even with an extender added, is now fast, snappy and practical... thanks to the new DPAF sensors on the newer EOS M6 mirrorless camera.
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It's inevitable that Canon's statement that "future photographers will use mirrorless cameras... with just one likely Pro-model DSLR for serious professional use"... will eventually become a self-fulfilling prophesy.   But if the goal is to take better pictures, this shouldn't be a concern.  It will likely take some years before mirrorless completely dominates. Prices will invariably drop and technology will be further refined.
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Personally, I really like the results from EOS-M series cameras with both native EF-M lenses and larger contemporary EF lenses.  The EOS-M cameras are also a lot of fun to operate, sometimes more so than using a DSLR.  Anything that adds fun to photography is going to result in people taking more pictures.
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Like it or not, mirrorless is here to stay.  Nikon are supposed to be rushing to put out a FF Mirrorless and Sony are still throwing money at the wall on the subject.  In fact Sony singularly blamed the Development of Mirrorless Cameras for almost bankrupting them in 2015.  Canon spent years indoctrinating the public into believing that they need a DSLR to take "Professional Looking Photographs" when a compact point and shoot was usually all they required.   Their DSLR and Lens Range are extremely lucrative to the Canon company.  It's simply a matter of time before Canon get around to releasing their first Full Frame Mirrorless camera. 
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Would I buy one? Probably.  I like the benefits of a FF sensor. And I suspect they'll throw in just about everything they can with their first model. 
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Some examples below from different EF lenses on the EOS M series camera...
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EOSM + EF 135mm f/2.0L USM - live performance - shot from the back of the theater.

EOSM + EF 100-400mm f/4.5-5.6 IS USM II + EF 2x III Extender

EOS M6 + EF 100-400mm f/4.5-5.6L IS USM II + EF 1.4x III Extender - taken 3 days ago.

EOSM + EF 100mm f/2.8L IS Macro USM lens - the insect is just 7mm long.

EOSM + EF 135mm f/2.0 USM lens - flash not required.

EOSM + EF 24mm f/1.4L USM II - also great for photographing the Milky Way at night.

EOSM + EF 50mm f/1.2L USM lens