While a QB may not change their mind mid-playaction, they will change their playaction timing based what they read pre-play. So if a blitz is coming, he'll shorten his playaction fake so that he can get the ball out faster. Or as well, if a guy does get free right in the middle of his playaction fake, he won't continue through the fake, he'll pull it and throw. I don't think it's really all that unrealistic.