Lets see the following scenario: average photographer wants to buy a camera and lens. Say he has several options, of course, and compares a FF DSLR and 24-105 f4 zoom kit, with a EVF FF and 24-70 f4 zoom kit. The former costs around EUR 2500, the latter around EUR 3000 (say a Canon 6D vs. Sony A7II). For some, that is a significant difference. Plus, traditional DSLR systems offer an incomparable variety of lens for expanding one's system. Remember, the photographer in case does not care about using adapters and so on. All he wants is to leave the shop with the feeling and guarantee that he made a good choice for his/her money. Arguably, DSLR systems are a very compelling option for our customer. If you go APSC format, the gap is even larger in favour of the DSLR. I mean, the other day I saw a kit with two zooms being sold by EUR 300...
Another scenario is the pro photographer, who only changes if he/she sees and feels there is a compelling reason to do so. Most pros I know actually value stability, which is a reason why Canon ergonomics have changed little since say the days of the Canon T90... if it works, don't mess with it. For Sony, this is a big challenge, as they are perceived as a company that changes products lines too much, without hardly ever completing one line, or fully committing to a product line. Canon EOS and Nikon have been around for decades, they offer reliability. Sony offers "innovation" and "fun" (or does it), but all could be over in 2 or 3 years (see Sony NEX line for example). Look at Samsung NX1 as another example; is anyone using it?
Finally, a third scenario is the "enthusiast" photographer, who wants to downsize the gear, but without compromising on quality. This is where Sony is having a success with the A7 series. A great combination of size, sensor quality, and some top quality lenses (with the help from ZEISS). And with (now) reasonable menu systems and ergonomics. This is what I have changed to in the last year or so, and it is working fine for me. I can carry the same quality with less bulk and weight. But to be honest, in the back of my mind, I still have a lingering doubt that Sony will "innovate" something in the future, and drop the A7 system... Again, look at Sony NEX line, it went the way of the Dodo for Sony APSC, not to mention the A mount system...