By 2040, many soldiers will likely use Virtual Reality and Augmented Reality headsets in various functions. Drone pilots may wear VR headsets instead of looking at computer monitors. Fireteam leaders will likely have helmet-mounted Heads Up Displays that show AR information about their surroundings.
But for VR and AR to become truly mainstream (ie usable enough for soldiers to use for hours at a time without side effects), issues with optics need to be overcome. One example of this is ‘vergence-accommodation conflict’ that causes fatigue and discomfort in the eyes after prolonged VR use.
In layman’s terms, if vergence distance and focal distance (see image above) are different in a VR headset, your eyes will eventually rebel against you. The video above goes in to further detail if you are interested!
The long and short of it: by 2040, companies like Oculus, Samsung, and Magic Leap will likely have collectively solved vergence-accommodation conflict. Soldiers and everyone else will be able to use VR and AR for prolonged periods — for better or worse.