The AF speed is comparable to EF lenses on live view mode on Canon bodies, the problem is the EOS platform is not designed for contrast AF.35mm and 50mm M glass mostly performs well on the a7R, for wide angles you have a huge range of lenses from Canon and Nikon that work on the body great. AF really isn't important for wide angles. (1)
The 35mm 2.8 is actually a great lens, it's comparable to the 24-70mm II 2.8 and Sigma 35mm in sharpness. Additionally, given this is full frame we're talking about, the performance in terms of noise, depth of field and angle of view is what a 22mm f/1.7 would provide on the Fuji X-Pro. I want the security of established platforms like Canon EF and I want to be able to use my EF lenses which I have no intention of selling as their performance is fantastic. I don't consider the FE mount interface to be something that provides much in the way of security or longevity. (2)
Yes, this is the use case for me. Landscape and architecture. Plus a general purpose walk about with best of class image quality (i.e. a Leica killer).
With the Sony a7R you get something that integrates well into your existing EF system and provides a few things that the Canon EF system doesn't do well. (3)
The Sony a7 is half the weight of any DSLR that offers comparable image quality, including the light 6D. It is also lighter than a Leica body. It is the smallest and most compact full frame body anyone makes. When you need it to be compact, you put on the 35mm. But you very much notice the smaller size even with big DSLR lenses. And this is advantage of the a7, it's a compact camera that makes no compromises. It's like a medium format that fits in your pocket. (4)
(1) To each one's own. My 50 and 35 asph do not perform well on the A7r, nor do my 21 SEM or 28 cron. I'm using EVF, not live view, for autofocus. Live view for me is a manual focus tool, too slow for how I shoot. Works for you, since you AF in live view and shoot static subjects, which is great ... for you. I disagree that AF is unimportant for wides, if they're at all fast and you're shooting close, as I do.
(2) All a matter of personal opinion and experience - my points were not meant to be absolute. You like adapting EF lenses and see none of the downsides of impaired AF speed using the VF because you use live view. I've experienced the sluggish AF coming from 1-series Canon bodies with EF lenses compared to an adapted 85L and 70-200 II on an A7r, no question for my use (VF based).
(3) Again, that's based on your shooting. Doesn't work so well for my uses. Doesn't kill my Leica either. Leica glass is much more enjoyable and consistent in performance on a M body than on the A7r, unless all you shoot is certain 50's and longer FLs. I have 3 Leica wides and a couple ZM's - none work well on the A7r, so I can't agree from my own experience.
(4) When I refer to small, I don't mean weight. I mean size. For me weight doesn't matter much. The A7r with EF lenses + adapter on it is just not significantly smaller than a 6D with the same EF lens. Not to my hands and bags anyway. And my hands and bags are not as full with a Leica M body and Lux 35 than with a A7r and 35 f/2.8, especially if the lenses have their hoods on. And my Lux is 2 stops faster than the 35 f/2.8 which should give the compactness factor to Sony.