I can't speak for working in VR but I think us desk jockeys tend to underestimate what other people are doing for work. The lack of movement is what does thought workers in.
There are of course many jobs that are much more taxing on the body overall than waving one's hands at a Minority Report style computer interface. (I used to work at a restaurant! I've done actual work, I swear!)
However, waving your hands in the air for 8-10 hours a day feels a bit unnatural relative to "actual" physical work. I don't have a real scientific basis for this statement.
But there are some physics at play IMO. In boxing it's generally accepted that a missed punch consumes about twice as much energy as one that connects -- your body must do the work to accelerate your arm, and then it must do a roughly equal amount of work to decelerate it. I think there are some parallels to waving one's hands in the air for 40-50 hours per week relative to "actual" physical labor.
More to the point: MR-style interfaces (at least as typically implemented/depicted) don't really offer tangible advantages to using a good touch pad. They trade tiny finger movements on a trackpad for huge sweeping arm movements that accomplish the same things.
I think if MR-style interfaces had somehow been invented before mice and trackpads, we'd be celebrating mice and trackpads as absolute miracles of efficiency.
I actually prefer keyboard alone due to the concise control, I find mice pretty clumsy. But there are things that it can do that I can't achieve on a keyboard and same for VR, perhaps we'll end up with blended work stations.
On the note of tiring out, I have been strength training for 12ish years now and I still get tired quickly when holding my arms above my head to work on my car. I think because they are a smaller muscle group, they can be saturated easily. I don't have the same issue with repetitive motions, it's just holding the arm in a similar position that does it.
Maybe if AR gestures take into account full motions rather than holding the arm in a similar position for too long, it might not tire so easily.
I have VR already and I will say it's an exhausting experience in general, I can flat screen game for hours, but in VR I want out in less than an hour. I think it's the full focus it forces on you, and the split world spatial reasoning going on.
