That 58mm f/0.95 is probably going to cost more than the D850 and the kit lens.
To me the only reason to upgrade would be for the new lenses and lens mount, whenever that becomes compelling. More frames per second isn't something that is important to me, so I don't really have an opinion on that. I have seen worse battery life scores, but that is not based on real-world experience and probably depends on use-case, but you can work around that. I have seen a lot of moaning about the lack of a redundant SD card slot as though that is the only weak point in data integrity chain from scene to secure photos.
I have a D750 and the lenses that I want and I get a lot of enjoyment out of this gear. To me, the new mirrorless cameras and lenses will probably be more compelling on or after August 23, 2020.