I'm currently holding off for reviews and tests, before buying the 200-600mm...but moreso I'm holding off in hopes of news of a new APS-C high-performance body - which is really more what I'm looking for to replace my excellent but well used and old A6300...If Sony release a 'mini-A9' with an APS-C sensor and the large battery, body controls, EVF, and focus system tech from the A9, I'd buy both that AND the 200-600mm lens together, and still keep the 100-400mm for maximum optical quality when needed, as well as the vastly superior close focusing ability.
Yes! A mini-a9 would be great. The a6400 was a bit difficult to use. Granted, I only had it for a week, but compared to the haptics and UI on the Nikon, I really preferred the Nikon. IMO, it needs both a front & back knob and a hard button to switch EVF/screen/auto. It also needs an easier way to switch between video and stills. For my hands, it is also a bit small in the grip.
If Sony could do something to compete with the d500, they would have a very popular camera.