A baby D5 would be a direct D700 replacement. It would use the same sensor as the D5 or the sensor to be used in the D5s. The D850 seems to be a new species that is both fast and high resolution.
I'd still like a D700 replacement that offers crazy high ISO with still decent image quality.
the iterative progress we're seeing in these DSLRs is remarkable > Adding and merging innovations into inherited features; in the case of the Df, the camera is built on the venerable ergonomics inaugurated in all the finery of the FM2/FE2/FA guild. The pro models beginning with the D3, and the D7xx, D8xx lineages are (have been) superb, truly.
Equally the D500 in DX. The latter being one remarkable camera.
hopefully free of glitches, the D850 should be a DSLR of Note ! These times we photographers live in have never been better :-)