Dedicated action stills cameras aren't dead, but their days are numbered. This goes as much for the mirrorless A9 as it does for the 1Dx2 and D5 SLRs.
Basically, they're being squeezed between general-purpose bodies at one end and video cameras at the other. With each new generation, the number of users who wouldn't do better with one of the other categories of camera diminishes.
It used to be that an action camera was necessary for shooting wildlife and other action stills. Other bodies simply didn't have the necessary frame rate or AF. And action bodies sacrificed a lot of resolution to achieve the needed frame rates. But we now have the D850 and A7r3 - at 9-10fps, they're as fast as action bodies of one or two generations ago, with credible AF systems. Not many action photographers need more speed than this, and many more could better use the extra resolution (for flexibility in cropping) than an extra 4-5fps. So, at the lower speed end, general-purpose bodies are now able to provide all the needed speed and accuracy of an action body, while being better in other respects.
At the faster fps end, action SLRs are running into video cameras. Video cameras always used to trade resolution for frame rate to an unacceptable extent for still photos - a 2MP 1080p video camera was no alternative to a 20MP 1Dx. But 4k video cameras are now common (fine for small print sizes or web) and 8k isn't far off. That's 39MP at a minimum 25fps. The exposure settings when shooting stills would be different from when shooting video, but the same camera can do both. And, with mirrorless AF technology having demonstrated tremendous advances this year (you can mention 'A9' and '1Dx2' in the same sentence without being laughed at, whereas you couldn't have said the same about the A7r2) the AF advantage of SLRs over video cameras is no longer an issue.
A lot of it comes down to frame rate being either an 'enough' or 'not enough' quantity, rather than a 'more is better' one like DR or resolution. Even if your scene only contains 10 stops of DR, a 15-stop sensor will give you less shadow noise than a 12-stop sensor, while, even if you only need 20MP, a downsized 80MP image will give you a much better picture than a native 20MP sensor. Conversely, if you need 10fps for the subjects you shoot, having 20fps isn't going to get you much other than twice as many files to sift through. Once you hit the required frame rate, you're probably better off improving other aspects of the camera rather than increasing the frame rate further. 9-10fps is probably a reasonable rate for wildlife, and most sports, photography. And, not coincidentally, that's where the A9r3 and D850 general-purpose bodies are sitting at the moment.
