When the D800 came out a bucket load of internet inadequates complained about the res and the inability to get sharp shots. I've used mine hand held for events, in landscapes with long glass, countless times on tripods and it just isn't true.
Bear in mind this is a camera aimed at either studio photographers or landscape shooters who probably have some Canon EF glass to use. I doubt this is anyones first Canon rodeo (bar potentially mine of I got a stonking deal on it). I've seen so many Canon stuff on workshops etc it's not a system I'd worry about using.
Consider the following:
A lot of my shots don't even utilize the full DR of what I have (the exposure sits comfortably within either end of the histogram). Particularly with Long lenses that frame a small part of an overall scene. I can think of many shots I've taken where I wouldn't even utilize the dynamic range of this 5dsr. In this case expose to the right, and bring the blacks back in post - quick slider in LR or do it in curves/levels in whatever you edit on.
However a lot of shots in landscape photography exceed that of the dynamic range of a modern sensor or even the medium format sensor in my 645z and that of the Nikon D850. What then - you just either blend exposures or use the grad filters that
@LongLensPhotography hates so much. I think they're fine but he hates them. That's fine. If you blend just buy a stack of memory cards and some extra storage for your PC.
What features do you think this camera lacks - and for £1290 on a grey site what do you think is better? It has a nice big ovf - decent AF (for static subjects anyway), live view (ok missing live view histogram - one feature I'd miss but metering is good on these cameras as are previews at the back give an idea of the histogram and exposure on R, G and B channels as well as overall luminescence values) - trust me - those who want this camera will understand these and will achieve optimum exposure second time around if they goose it first time around.
Given the res, the price and the vast array of used/new EF mount canon glass out there this camera is sound for 7yrs.
Here is the thing about Sony cameras, and mirrorless in general that I find makes things hard.
1. They feel small and fiddly. The best one handing wise is the Panasonic S1r. It's nice a and big. With a big lens on they don't handhold well, and on a tripod can tip forward. A tripod collar for a heavy lens fixes that though. But I cannot abide the small size.
2. The big one. They let in f*** tons of sensor dirt. I mean all the time. You will spend ages in PP cloning it out - and don't think LR catches them all with it's autoremove thing. it doesn't. You'll spend longer cloning out dirt than actually doing your RAW development. If you travel away and change lens a few times and haven't got access to a dust free place to clean the sensor, that will be annoying and in some cases detrimental to your out put. The 5dsr is properly sealed, if you change the lens you have the mirror in the way. It takes many lens changes before it's grubby in there. And for the price you can buy two of them almost for the price of 1 a7r4.
3. The EVF - some love them, others hate them. Guess which camp I fall into
4. Your point about the multi lens adaptability is valid - but with all systems the best ones are probably native to the system so you just use these. And how many lenses do most have. I am down finally to 4 and it's liberating.
5. Google PDAF banding.