The recent f/1.8 Nikon series...which by all accounts are optically very good... could be substantially smaller if they weren't AF.
Tokina announced an all metal manual focus, manual aperture 20mm f/2 lens today for Sony full frame mirrorless with a 62mm filter thread weighing 490 grams. Compare that to around 350 grams and a 77mm front thread of the Nikon 20/1.8. So it could easily be a
choice between weight or size. (The Tokina Firin 20/2 is for mirrorless only. It has got a CPU, so it's a Sony mirrorless equivalent to Nikon Ai-P, though with stopped down metering.)
Maybe a new f/2.8 line could be made as an old school MF lenses, but with a CPU. Btw. Nikon Ai lenses have nicer build than Nikon Ai-S, and may have a more generous focus throw. Zeiss does it, so maybe Nikon too?
Manual focus is by no means not out of fashion.