Just wanted to add my two cents to the struggle. The Mpx fight seems to be an endless debate, nevertheless, I am always surprised to see people who seem to decide what is good for others. More resolution will always be wanted, that doesn't mean it's always needed. Saying that more pixel is useless is just nonsense; it might be useless for you, but some folks need more resolution for different matters. I am old enough to have worked with film for a good while, and in the good old times, everyone was trying to switch to the newest film because of it's better sharpness and finer grain. Some jobs needed better resolution than others, a press photographer could deal with small format (24x36), while fashion required medium format and advertising large format view cameras; nobody would discuss if 4x5 format was useful or not. It's like asking Ansel Adams if he really needed to shoot 8x10 sheet film.
Nowadays, things have changed a bit, since all DSLR outresolve easily small format, and the only thing that is still out of reach is the large format (minimum 4x5) cameras. On the other hand, most people only watch their pictures on screens, so for that purpose, most DSLR are overkill.
But that doesn't mean that there isn't a demand for higher quality. My work is architecture, landscape and interiors, where every pixel available is required. These jobs were previously done ideally with large format view cameras, one of the domains were DSLR cannot still compete.
One of the option is to put a digital back on a view camera, but we talk about 30 to 50k budgets here, plus they are extremely awkward to use in the field. The cheapest modern medium format back is worth 20k for 40-50MPx resolution, so the idea of getting 36 Mpx for 1/10th of the cost makes perfect sense.
Sure not everyone needs that, as a hobbyist, if you take pictures of the dog in the garden to show on facebook, it's just plain ridiculous, but for lot of working pros with budget limitation, the Sony 36Mpx sensor is plain gold, whether you are a Nikon user (D800) or for us Canonists in the form for now of the Sony A7r / metabones combination. The Sony has the immense advantage of "opening" the system, and to get rid of the dependence to one lens/camera brand.
You like it or not, the Sony sensor is more advanced than anything Canon is able to put on the market. It's one thing that you are perfectly happy with your equipment (or for some that you rage about the inability from Canon to design a competitive sensor for now), but it is another one to say that nobody needs better. I can read that lots of people here use their camera hand held, focus with AF and make extensive use of high ISOs. For this use, a 1Dx, 5D3 or 6D make perfect sense; in my case, I use a tripod 99% of the time, very seldom use more than 400 ISO, and focus manually since 75% of my work is done with TS-E lenses.
I could easily say : "why the hell do people need 150 points AF, 12Fps and 12800 ISO"?. I don't, because I know some folks shoot different things than I do, use different technique and simply have different needs. So maybe it's time to admit that other people may need more than the average 20Mpx than Canon can offer us nowadays.
It's not because you don't need it than no one does. I work with a 5D2 and I've reached the limits of its sensors, some of my clients would like (need) more, but the 50k digital view camera option is not a realistic financial option in my case. Fact is that Canon has nothing better to offer for my needs than my 5 years old 5D2. So the option will likely be the Sony / metabones combination. There is no shame here, and I consider myself lucky that Sony is offering an alternative.
When I shoot for pleasure, I use a Linhof 4x5 camera with Schneider lenses, and the digital files I get from my Canon simply look ridiculous compared to a well scanned 4x5.
A more careful a way to put it would be to ask in what case more resolution is needed, instead of assuming that what is good for you is enough for all. To finish, I do not agree with the idea that the D800 (or A7r) are niche products; they are not for everyone, and have not been designed as all-round cameras. Nikon has the D600 and Sony the vanilla A7 for that purpose, but that doe not make them niche cameras, simply specialized ones (ask pro fashion and beauty photographers if their Hasselblads are niche cameras). I think it is just different approaches, Canon wants every camera to be an all-rounder, but doing that, they have pushed away specialized users. Nikon or Sony make different cameras for different uses, and I am betting that the new Sony A7s will be extremely successful with videographers, though as well a specialized camera.
