Thanks Kcarbotte for all your very insightful comments and answers.
Regarding VR Sickness, I would say it is the crux of the whole affair.
This matter is extensively studied internally by Oculus VR and also on forums :
https://developer.oculus.com/documentation/intro-vr/latest/concepts/bp_app_simulator_sickness/
https://forums.oculus.com/community/discussion/170/virtual-reality-motion-sickness-vr-simulator-illness-guide
The bottom line seems to be this : as long as the human brains take visual cues from movement, but all other bodily "sensors" do not record movement, the brain interprets this as some kind of poisoning by neurotoxins and triggers the nausea/puke reflex.
The only workaround would be extremely efficient Omni directional Treadmills or 1:1 simulation space.