I think stamina is playing an important role in game shape calculation but I don't understand why would lack of minutes or extra minutes guarantee decrease of game shape while optimal playing time does not guarantee increase of it?
Whether there's a single point of perfection or a range, the closer you are to it, the more likely you are to increase or maintain game shape. Equally the further you are away from it the more likely game shape is to fall. There's no 100% guarantee either way, but people tend to assume far too many minutes is so likely to hurt game shape that its safe to just say its guaranteed.
Also, training of game shape has become almost useless.
Sadly, not true, but I'm happy that its less useful than it was.