I think both your premise and conclusion is wrong.

You make it sound like VR motion sickness is the norm, but it’s really not. Tons of people play VR games every day with smooth locomotion, on “potato VM” game engines, at 72 or 90 Hz without issue.

Even if perf were a widespread issue, building a custom game engine for VR probably shouldn’t be your first choice when there’s so many easier ways to optimize.