True there was plastic bottom and prism for the outer shell on the F4 - but for sure the whole chassis was magnesium!
I think you misunderstand, I actually see the benefit of a outer shell in plastic and rubber to protect the inner magnesium chassis the lenses work really well with that design as well!
What I'm trying to say is that it is super important the lens stays on the camera no matter what happens, more or less, otherwise you will have to switch to the back up camera, not always easy for a PJ, event type shooting situation where your running around,,,
I was shooting the F4E,,, BTW A true picture machine - I don't think I ever had a bad image with that camera,,,
What could be a way to use plastic in a safe way for the case of the D810 design, could be if they changed the screw design: Beef up the design with a larger diameter screw, double the diameter to 5mm or something, so the screws really could connect the F-mount to the mirror-box - A slight change to the screw holes in the mount would be needed nothing else,,,
It looks like they ignored this and just used same size as for magnesium, just like they do on the plastic lenses,,, It is the initial rip out of the first screw that cascades into a total failure,,,,