D810 is made compact and economical - with interchangeable viewfinders it would not be compact nor economical (especially if the EVF is really cutting edge, which could cost many thousands just for the viewfinder). If it has a fixed EVF then it would upset those who want OVF (certainly myself). A D5 doesn't aim to be inexpensive nor compact, so interchangeable viewfinders would have been conceivable (looks like only the eyepiece area is removable, I don't know why they would do that) and they could make a high res variant of the body ("D5X") based on the same chassis and using the same viewfinders. The viewfinder options could separate it sufficiently from the D810 so that it would sell in acceptable numbers. There could be numerous options for the viewfinder.
For video purposes which I assume to be common application for a D5, an EVF would be useful and it could give the camera an edge to compete with Canon's Cinema EOS products.
Personally I don't want an EVF for still photography and I don't do much video, but I would like everyone to be offered something. And there have certainly been requests for Nikon to make an FX/DX camera with EVF. If it is an accessory, only those who need it would have to pay for it.