Qbs should never get tired....they rarely run the ball.
Fatigue scales need to be looked at. It's that simple. QB should have arm skills nullified till they get warm . In other words drop that accuracy and arm strength significantly for two or three passes.....unless the starter is actually injured .
Back to general fatigue , imo it should tied to the bmi of a player . Weight divided by height . Then have players ability . ....all of them fall off a cliff. First speed and strength should have a general linear decline , but at some point in there all skills just don't work.
It should be visible and we should see a notification , perhaps in the log, that the player is winded and is signaling to be replaced.
Our guys are full go all the time....so many pros take appropriate breathers when a play is " out of reach" ........or they just get beat / picked on .
Also, I am not sure if the time between snaps is accounted for here. As it varies irl....and that recovery time is essential for every down players.