Ok, your statement might be right for an amateur league (in which I am probably playing) but I still defend the fact that as an athlete you get stronger, more experienced and more motivated with each game, and on the other hand: what is "stamina" then standing for, if game shape is the fatigue?
I mean, it's not that the players are thrown into the game without any training! They train basically the whole week, so they should be able to take a game's pressure and still bring an excellent performance. To say an athlete gets tired by playing three games a week, while he has practice the whole week doesn't make sense to me.
I think the BB have sort of blurred the difference between GS and fatigue. A few seasons ago, stamina would drop if a player would play too few minutes. An injured player might drop several levels. The problem was then that you could only improve stamina with team training - even though stamina is probably the skill that can best be improved by individual effort.
So that aspect of stamina was eliminated, and the current system where there is only a long term decline was instituted. You still have a problem where you have to train everyone to improve a couple of players.
Realistically, fatigue should carry over from game to game, so that if your player has played 40 minutes 3 games in a row, he will be more tired than a player who has only played 40 minutes in one game. So the game shape drop for too much play sort of represents that effect on a delayed basis, and he can recover by going back to a more normal amount of play.