RObert Budding
D'oh!
This is the future: Nikon and Canon are not rushing into an already crowded "niche" mirrorless market; by seriously committing the best of their resources to the new endeavor (they had to, as the new cameras will face imminent competition from the now formidable a7 cameras), they are turning their entire DSLR business with most of its existing customer base into the mirrorless. They will make the transition, the mirrorless market will dominate the scene, and DSLR will be dropped completely someday. Bodies like the a99 II will be released so those who got left behind could have a taste of the cutting edge, but no more lenses. No matter what Nikon says the D850 could very likely be digital F6 - the last of its kind.
Ditching the mirror box and saying goodbye to a 60-year heritage is a big thing - like SLRs taking over rangefinders, or the advent of autofocus. Only this time Nikon learnt to wait long enough that it's their customers actively calling for a new mount - and eventually getting what they asked for, so they did not "betray" F mount users like Canon once did to FD users. Almost cost them everything, but that's very Nikon-ish as well I'd say.
I seriously doubt that the D850 will be Nikon's last professional DSLR. There is a massive installed base of F lenses, and there is no way that Nikon would stop selling bodies to willing consumers. Besides - dropping new DSLR bodies would only encourage existing clients to look at brands other than Nikon. Ain't gonna happen.