Hi,
I agree with you.... With a full frame mirrorless, I'd rather it just accept existing EF lenses without an adapter. However, that ain't gonna happen according to the rumor websites. We'll need to use an adapter.
Rumor has it there will be multiple adapters because one is plain, while another is a neat design that accommodates drop-in filters. Clever idea that will make possible (easier and less expensive) to use filters with lenses like the TS-E 17mm f/4L and EF 11-24mm f/4L USM, which have a strongly protruding, convex front element. Right now the only way to use filters on those is with a filter holder that clamps onto the front barrel of the lens and oversize square or rectangular filters. That's a pretty large, clumsy and expensive arrangement. (Note: Some lenses have slots on the rear for gel filters, too, but those are fragile and can be hard to find.)
With the M-series APS-C format mirrrorless cameras, it's water under the bridge. Adapters are already a fact of life, if you want to use EF or EF-S lenses on them. Designing mirrorless with an ultra short lens register (and hence, require adapters) appears to make more sense with APS-C models than with full frame. That's because there's a lot of size/weight savings to be had with the crop sensor cameras. There's some similar savings, but not nearly as much to be gained with full frame models. Their lenses, in particular, end up being virtually the same size and weight as DSLR full frame-capable lenses.
However, a nice thing about the short lens register is that adapters will be available to allow use of other system lenses as well as a huge array of vintage lenses, many of which can't be used via adapters on a modern DSLR. Add the fact that a mirrorless camera's electronic viewfinder can provide helpful assists for manual focusing (such as "focus peaking") and you've got a pretty interesting opportunity that didn't exist before.
Mirrorless cameras are naturally harder on batteries than DSLRs. That's because the image sensor and electronic viewfinder need to be active and drawing power continuously while the camera is in use (if not, the viewfinder is blacked out). In contrast, the optical viewfinder of a DSLR doesn't require any power. Both types of camera draw power while active, in order to meter, run autofocus and image stabilization, and of course to take images. But the heavy power drain by mirrorless continuous use of the image sensor to capture what's being seen through the lens and then project it on a little LCD screen in the viewfinder is in addition to all the other battery power usage that both types of camera need to do.
Another way of looking at it... A mirrorless camera is much like a DSLR in Live View mode. For example, look at some of the existing Canon M-series and Canon DSLRs that have similar specifications and share use of the same LP-E17 battery. The Canon Rebel T7i/800D is rated to take 600 shots on a battery charge. The Canon M5 with the same battery and is rated to give 290 shots. But in Live View the T7i/800D drops to 270 shots per charge. Shots per charge ends up being almost identical, when the DSLR is using it's rear LCD and keeping it's image sensor running continuously, to provide Live View.
I would like to see a battery grip made available for the M-series cameras. (I believe one is planned for the full frame R, we'll know more after the announcement this coming Wednesday.) Those grips not only double battery capacity, they also provide a convenient grip and secondary set of controls when using the camera in vertical/portrait orientation. (I use grips on all my cameras.)
I've found that the "rated" shots per battery charge are usually artificially low. The rating is based on a CIPA standardized testing procedure, which calls for flash to be used on half the shots (on cameras that have a flash built in). Simply by not using the camera's built-in flash will greatly increase the number of shots per charge. (Built in flashes are pretty wimpy anyway.... a separate accessory flash with it's own power supply is better in many ways.)
And there are other power-saving tricks you can do. For example, I set my cameras so they don't automatically play back each and every image right after it's taken. Every so often I'll call up an image to check exposure, composition, focus, etc. Reducing use of the rear LCD screen significantly increases the number of shots per charge. I also turn off GPS (and don't have, want or need WiFi on my cameras). And I set the camera to go into sleep mode quickly.... This is not a problem because today's cameras wake up nearly instantly (it was more of an issue with older DSLRs... I missed shots waiting for them to wake up!)
Using all these simple "tricks" I get pretty close to double the rated shots per battery charge with my DSLRs. There will be less opportunity to do that with any mirrorless, because they have to run the sensor and the EVF continuously while being used. So the best thing to do is carry extra batteries.
I have no doubt that Canon will continue to develop and make DSLRs for a long time. After all, DSLRs are still the top seller, in spite of the popularity and growth of mirrorless. There are things that DSLRs do better.... Just as there are some things that mirrorless cameras do better. There are pluses and minuses to both types of cameras.
Personally I will keep using my APS-C DSLRs for sports and a lot of wildlife photography. And I plan to keep using a full frame DSLR for some other things (macro, portraits, scenics). But I plan to get an APS-C mirrorless for use with several manual focus lenses, as a fun, compact and unobtrusive, "street photography" type camera.
Sorry, I can't seem to be brief! 
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7DII (x2), 7D(x2) some other cameras, various lenses & accessories
FLICKR & ZENFOLIO
