The only killer now would be a new Sony A7r3
I doubt it. Unless the prospective specs of the A7r3/A9r are way off, Sony would have to introduce another line between the high-speed, low-resolution A9 and the low-speed, high-resolution A7r bodies to compete.
A 70MP, 5fps-or-slower camera isn't going to touch the D850, even with the A9 AF system. All it would do is peel away some of the studio/landscape users - it wouldn't have the same general, multipurpose appeal of the D850.
A 60MP, 6-7fps A7r3/A9r would do better, and may be Sony's best option if it doesn't plan on producing something between the low-resolution/high-speed and high-resolution/low-speed models.
A 50MP, 8-10fps body would provide a true competitor to the D850 (provided Sony keeps expanding its lens lineup - both Canon and Nikon will have to do the same when they launch their mirrorless bodies, although I'd imagine Nikon would be quick to bring out a mirrorless version of its spectacular 70-200 f/2.
. But it's also a fairly unlikely resolution for the A7r3/A9r - almost certainly, it'll be something higher-resolution and slower.