For me it's all about the amount of MP ... for instance I shot
this on my 7D MKII which is a 20MP crop sensor. the picture is then cropped to over 100% which left me with a 300 KB file, I would have liked to have gone in more but it just turned to mush. If I had done the same on the a9 which is half the MP in APS-C mode it just wouldn't have stood up at all. Ideally I would just get the A1 but I'm not prepared to spend 6k on a camera body.
Apart from that it's a fantastic camera and the 200-600mm is unbeatable in any other system IMO.