That may be the cases in training scenarios. Actual war is different.
In actual war, the side which has the most people able to do the most difficult challenges, is more likely to win. The side which is unable to match or beat their opponents' capabilities, will die.
You are 100% correct. I am not talking about war, I am only talking about how they would devise resources needed for a task, and training people in that task, which would translate into a wartime portion of a mission.
We all know that once a fist hits you in the face, all the planning goes out the door.
How the scenarios translate (This is just my opinion) is that if, in a combat situation, you need to pour 60 rounds per hour. from a gun that has to be fed by a single soldier hand carrying munitions over 20 yards of terrain at the rate of one per minute, but the person on that crew carrying can only do only do 1 round every two minutes and tires out completely and is unable to perform after 10 min at half the rate, a fire support mission is going to fail, and the soldiers on the other end depending on it may fail as well, resulting in injury, death, loss of engagement, battle, and war.
And that doesn’t even take into account ALL the other things outside of task above, hand to hand combat, or anything else with a high dependence on physical exertion.