Therefor for serious use I postulate some things which have to be happened until a Replacement of the mirror makes seriously sense:
- adequate electronic wysiwig-viewfinder
- permanent autofokus in the quality of the actual systems, incl. sensitivity under low ligh
- compatibility-bridge to the existing system
- new body-desing to bring serious advantage to the user.
On all four points above ... micro-4/3 is check, check, check & check. Mirrorless technology is here, now and fully usable. Conclusion? Canon is falling behind, as is Nikon, with only their lenses keeping the (obsolete) cameras afloat.
re. size/weight advantage of mirrorles vs. DSLRs: yes, tele-lenses and tele-zooms like a 70-200/2.8 L IS will be the same size and weight and their use will almost nullify the size/weight advantage of a compact FF-mirrorless.
BUT unless totally specialized in photography of certain sports or wildlife, most photographers will not use tele-zooms or long tele lenses ALL the time on their cameras. Actually, many photographers will never use such lenses. Probably 99% of all images are captured using focal lengths between 24 and 100 mm. These lenses especially wide-angle for a FF camera body with a short flange back distance could be considerably smaller than current EF-lenses. Point in case is the Leica M-system and its "surprisingly small" (fixed focal) lenses. And contrary to common belief, adding a ring-USM AF drive would mean very little additional weight and bulk, since movable lens mass is quite small in these lenses. IS would ideally be in-body IS. Viewfinder image on an EVF can be stabilized by purely electronic means. Using a few clever algorithms and ample procesing power, legacy EF (tele) lenses with IS would work in tandem with the in-body IS to give up to 5 or even 6 stops total stabilization effect.
That would finally yield a really small and light kit for the many occasions when we want to go small and light without sacrificing anything in performance, speed, ergonomcis and IQ compared to a good but big DSLR. The only limitation would be available tele-range in native-mount. Only when we need more tele range will we then pack and carry a simple and cheap-to-build extension tube adapter without optical elements plus any existing EF-lens (tele/zooms). But only then. Not all the time.
This is what I am waiting for. My current 7D plus EF-S and EF lenses is my last DSLR-based system. I want and will "upgrade" as soon as I get a Canon EOS 5D-M with a mirrorless body only slightly larger than a Sony RX-1 - to accomodate a built in Hi-End EVF. With a new sensor with ultra-fast in-plane phase-AF of course and an image processing pipeline that at least fully matches the current Nikon D800. Along with in-body IS and built-in WiFi, GPS and EX-RT wireless flash radio commander ... these radio components can be had ridiculously cheap and small. Price? Clearly below a 5D III, since it is so much cheaper to make a mirrorlss body without all the hi-precision mechanical cr*p in it - mirror, sub-mirrors, large and expensive glass prism etc. And, Canon - please aslo do away with that mechanical shutter and start using fully electronic shutters with X-sync all the way to 1/8000s.
And put an "as large as possible" fully FF-capable lens mount up front. Along with a number of "as small as possible" FF pancake AF-lenses (think of the EF 40/2. between 20mm and 85mm [20/2.8, 35/1.8, 50/1.4, 85/1.. Make them AF-only. Forget about those manul focus rings and gears. I don't ever use 'em. That way, it will be smaller, lighter, easier to fully weather-seal and lower cost. And for conven ience two hi-quality, ultra-compact "folding" zooms: a tiny 17-50/4 kit zoom and a 50-150/2.8 even smaller than the former Sigma 50-150/2.8. And the EF-adapter. That's all I need. :-)