Personally, I've taken about 55,000 photos at ISO 800 - 3200. I've taken about 15,000 at ISO 400, and less than 10,000 at ISO 100 and 200. Of the ISO 100 photos, I have needed more dynamic range than my 7D offers in about 2000 shots, however I am usually short by maybe one stop (and that is more because of the 7D pixel size...if I had a 5D III, I would have what I need for pretty much everything I've shot before.) In the cases where slight vertical banding noise did show up in the shadows (maybe a couple hundred at most)...I used Topaz DeNoise 5, and was not only able to remove the banding, but I also gained more dynamic range (that's what happens when you reduce noise anyway...you gain DR, but Topaz has a feature that attempts to further recover DR that was lost to shadow noise due to a loss of tonal fidelity, which gains me even more.)
I use GND filtration for my landscape photography, so dynamic range is actually something I have a lot of control over in the field. I would actually greatly appreciate more native sensor DR, as it would reduce my need to use GND filters. It would also help me avoid that unsightly GND artifact where mountaintops end up dark or even black when you need to use more than two to three stops of filtration. That is the single situation where I think having more dynamic range would actually be the most important factor for IQ...ONE situation. I also suspect that tonemapping 14 stops into 8-10 stops without ending up with quirky shifts in contrast and color fidelity would still be very challenging, and I highly doubt I would stop using GND filters even if I had a D800. I still doubt I would push shadows around more than 2-3 stops....but it would be 2-3 stops along with fewer GND filters, which still makes the job easier in the end.
I've been used GNDs for 20+ years, but that's because I have to. However I would not say that gives me control over DR. It allows me to compress it into a range which the sensor can capture. But if you didn't have to use a GND, wouldn't that be better?
Re low ISO - surely the point is that where possible, post processing should be kept to a minimum. It's not the fact that you can push, it's the fact that if you could avoid it, then you can spend more time doing the things you want to, and less effort after.
Third, the more you can see the picture as is, in the field then the less you have to visualise what post processing will do for you.
As a landscape photographer, I would like lower ISO (<100, ideally ISO 12 as I used to shoot in slides), and yes I would like the ability to have better DR and less noise in the picture full stop (but there are also a lot of times where you don't want greater DR and you will reduce it to focus the viewer where you want to)
When I do wildlife, urban or sports the same is true. But that does depend on the shots you are trying to take.
However, does the 1Ds III still take amazingly good photos? Sure does. As does every camera in the last 5-10 years. But is not the point that we still want better/improvements, and aside from the UX/Software features, where else can Canon improve (AF for movies, DR, tracking algorithms)? Like you, I'm not saying it's the end of the world for my photos if I don't have these features - you work with what you have, it's just that it would make it easier.
As for stats on ISO ranges - surely that changes with time? With the 10D I hated going to ISO 200, let alone 400. With a camera today, I'm not bothered about ISO 1600. So I'm not sure that's too helpful.