http://www.imaging-resource.com/news/2017/11/13/sony-interview-photoplus-expo-mirrorless-strategy
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1. We always listen to customers feedback
2. Only few customers asked for a resolution increase. That’s why the A7rIII still has the same Megapixel number as the A7rII: “yes, some customers want even higher megapixels. But a majority of customers requested different areas of development such as longer battery life, for instance or faster autofocus, or even the picture quality in low-light conditions. So considering that kind of feedback, we decided which way we should go for the third generation, based upon the customers of our Alpha 7R-series.“
3. Technology development moves faster than expected so customers sometimes don’t ask for certain things sony can actually do
4. Unlike the A9 the A7 series is made for a wider range of customers (male, female, young, senior). And for that range body size matters.That’s why the A7rIII kept the same light size/weight.
5. Sony did decide to NOT adopt XQD cards because accessibility of those cards is still limited"