For optical viewing there is the issue that faster mirror movement introduces a louder sound (emphasized by the higher frame rate) and more vibration or less viewfinder and AF system "ON" time. There are compromises involved. In mirror-up or mirrorless operation, it is possible to increase frame rate as technology progresses. Personally I want to do more by shooting single, well timed exposures to reduce time spent editing and to make the process more personal, less daunting and more effective. I get it that some phenomena are so fast, high frame rates are needed for consistency in capture but that's not really my thing. I'm more interested in human expression of emotions and for that single shots work best, and even for most movement I choose a much slower frame rate than 12 fps. The D850's 9 fps seem perfectly fine as the upper limit to what I might need. Mostly still I avoid continuous shooting and prefer to shoot what I could predict coming.