Leica seems also to use the sapphire "glass" for their rear screen (on their "improved models" Leica M-P or M10-P).
Wait and see if Nikon will use sapphire on future lenses, but if it is hard and solid, why not as a front element of long lenses or fisheyes and super-wideangles ?
Sure the price tag will be as unique as the lenses (remember in the 70's, the first Zeiss 250 Apo Sonnar, Leica 180 Apo Telyt, Nikon ED lenses or Canon Fluorit lenses...?)