Your understanding of it seems to be based solely on recovery from being tired. It isn't about being tired. Your body (legs, ankles, knees, practically every joint) only has so many miles in them, just like a car. The concern isn't so much that Bishop is tired at the end of a game. It's that by the end of his high school career, he's going to have been run into the ground before his real career ever gets started.
I can't believe I'm having to explain that simplistic idea to you. And it isn't 40 snaps a game he's seeing. It's 40 carries. 40 times he touches the ball with 11 other people intent on piling on him, diving at his legs, taking shots at his head/neck/shoulders. Injury risk aside, that kind of beating repeated over 40 carries, not to mention the other snaps where he is either running routes or blocking, takes its toll.
And yes, his young body can take it to an extent. But he will pay for it toward the premature ending of his career because he was run ragged as a youth. There is a reason college and pro coaches manage snaps and you see so many shared backfields at that level now.