When Canon introduced the EOS 5DS and 5DS R, there was plenty of reaction from the marketplace. If you believe the expression that a good deal is one that makes everyone unhappy, Canon hit a home run with these DSLRs. Complaints came from filmmakers who wanted more motion features, action shooters who wanted more speed and landscape shooters who wanted higher ISO. In fact, Canon added two DSLRs to their EOS 5D line that complement the existing EOS 5D Mark III beautifully.
Myth: Perfection Exists.
Sony is using a similar strategy with their a7 full-frame mirrorless lineup. The a7 Mark II has moderately high resolution and solid motion-capture features. The a7R is a high-resolution model with a 36-megapixel sensor (as of press time, the Great Rumor Mill that is the Internet is forecasting 50 megapixels, with a chance of more megapixels coming by summer of 2015), and the a7S is a low-light and 4K motion titan. Everything is a trade-off. You can’t have the a7S’s max ISO of 409,600 on a full-frame sensor with 36 megapixels…at least not yet.
The current trend is to extend camera models horizontally. This is much like car manufacturers who add different trim levels to a model to emphasize economy, luxury or performance. Continuing in this vein, camera manufacturers seem to be creating their lines so photographers can identify where they want to be in the overall hierarchy and then adding other trim levels. It’s like you decide that you generally need a 5-series BMW and then you can choose the 528, 535, 550 or M5.
Automotive analogies aside, the simple fact is that there really isn’t one ideal camera for all possible situations and styles. We don’t all have the budget to purchase an infinite number of cameras so we make compromises. And compromise isn’t a bad thing. Any device that tries to be all things to all people is likely to be good at nothing. Think of a car-boat. It’s probably not going to be a great car and you don’t want to rely on it to float. (For a great "real-life" example of this, see Top Gear, Season 8 Episode 3, when Clarkson, Hammond and May were challenged to modify cars to cross the English Channel.)
For professional photographers, the ideal camera is the one that makes the most money for you. It could be a $400 GoPro if you’re creating crash photos and videos for clients. Or it could be a $40,000 medium-format camera with a 150-megapixel back. Instead of getting hung up on juggling the specs in a comparison chart, choose a camera that does the job and add others as you encounter clients with other needs. A lot of professionals will find that they might start with a Sony a7R for its resolution, then add an a7S when they have to work in low light or deliver some 4K video.