The biggest problem with "game changers" is that change can be good...or it can be bad.
If we are 30 points ahead (oh, how I wish), then why would you need a game changer?
If we are 10 points ahead (oh, how I wish!), then we bring on the game changers and we lose, we changed the game for the worse.
If you've been on the pitch for 70 mins slogging your guts out, and someone comes on as a game-changer, what does that make you feel like?
There must be a better option...