I think the lenses are an issue. Sony thinks the lenses are an issue, that is why they are designing completely new types of lenses to work optimally on a hybrid Sensor-PDAF/CDAF cameras. For example Sony FE Sony 70-200/4 is totally different type of lens than prior Sony A Mount zooms; even the optical construction is optimized for hybrid-AF (dual linear AF motors).I don't think lenses are an issue for Canon in regards a mirrorless body. A lot of the A7 buzz is based on Canon lenses working quite well on the newer bodies via the metabones.
A7R II is ok or even good (not great, excellent or full-featured) with adapted glass, all cameras prior to it were absolutely rubbish (personal experience since Nex-7 and Metabones Mark II). Native glass focuses noticeably better on low light (it can switch seamlessly between PDAF and CDAF) and it has features like Eye-AF and Lock-on AF that adapted glass cannot use.
About mirrorless AF challenges and why hybrid focus (that needs optimized glass to function fully) wins let me quote Imaging Resource interview of Kimio Maki, Senior General Manager, Digital Imaging Business Group, Sony Corporation.
http://www.imaging-resource.com/news/2015/06/16/sony-qa-the-must-have-sensor-tech-of-the-future
DE = Dave Etchells, KM = Kimio Maki
DE: What about the low-light limit when you have phase-detect pixels? It seems to me that the separate phase-detect sensors like in an SLR, they can have big pixels, but on the sensor chip, we have, I mean they're limited to the size of the actual pixels.
KM: That's a good question. So, when we use... in terms of autofocus, we are using a hybrid focus, so first we use the phase-detection focus to get close to the object. Then near the object, we start to use contrast AF to get a clear peak. Then we adjust the focus. Then in order to make very precise focus (because 42-megapixel means very very small pixels), and also the focusing point could be a very small one, therefore we phase-detection together with contrast AF, contrast focus.
This graphic shows the process Maki-san was talking about above. Phase-detect AF gives the camera a good estimate of the correct focal distance, so it can command the lens to move to very close to the correct final position. Then, a brief contrast-AF cycle finds the precise optimal setting.
DE: Right, right. And does that help you deal with lower light levels, as well?
KM: Yes, yes.
[Ed. Note: Again, there was some back and forth here that was pretty fragmented, so I didn't include it. The central point seemed to be that, especially in low-light situations, the phase-detect AF wouldn't be accurate enough by itself, so they use a hybrid of phase-detect AF, to get into the right ballpark, then contrast-detect AF, to find where the actual point of best focus is.
This answered a question I'd always had about on-chip phase-detect focusing, in that the image sensor pixels are quite small, and have to be read out quickly (meaning must have a short exposure time), to have a responsive AF system. The phase-detect pixels are also shaded, to "see" only light rays coming from one side or the other, so they're only getting half as much light as normal image-forming ones. Under low-light conditions, both factors result in a small focus signal with lots of noise on it. Contrast-detect AF is more capable under those conditions, because it's looking across a larger number of pixels to develop it's goodness-of-focus signal.]
Based on how the number one sensor-manufacturer tackles these issues in their 3rd gen on-sensor PDAF cameras I do not see Canon all of the sudden solving these issues that have background on very fundamental physics; and Canon has not really brought anything new into sensor-development outside resolution in last 4-5 years. Not to mention their first attempt in on-sensor-PDAF (the Dual-pixel AF) was quite terrible.
They could release a mirrorless EF mount body; one that would be significantly thicker than current Sonys (flange distance difference alone is 26 mm), mostly and with lesser AF since most of their lenses just are no good for CDAF. So we would have a DSLR-sized camera with Live view and low light AF and other AF features quite propably worse than current Canon entry level DSLRs.Canon already have a massive high quality and fast lens line up. They just have to release a kick *** mirrorless body in EF mount (or with functionality) and we're done. Imagine if they did drop a mirrorless rangefinder into the market with 2.8 zooms available and 1.2 85's.