Any predictions for how long the F-mount will be around (meaning still in general use for the flagship Nikons without an adapter)?
I suspect it will be when the first mirrorless flagship camera is released, and that will be waiting on a good electronic viewfinder. My guess is 15 years, but the good news is that with an adapter the F-mount lenses would work just fine on the future camera. They probably would still produce telephoto lenses with an F-mount because the design doesn't change much, but for wide angle it would make sense to use the new mount.
Also, do you think they would stick with the same imaging circle or go for a bigger one to accommodate larger future sensors? The issue there would be compatibility with existing F-mount lenses, but they could still work in an FX crop mode (even a square 36mm x 36mm crop mode) like the FX cameras do DX crop.