Barkley simply doesn't have the football brain required for the top level. You cannot teach a football brain. Rooney had it when he was 16 and Ross has always acted on instinct when he is at his best. The moments he has to think, he is useless. Having two feet, strong, quick, etc, only gets you so far. If you don't have it between the ears I don't believe this is something which is gained through experience.
Koeman, I believe, will replace him when he gets the chance. It's basic errors with him which appear consistently. This belief he is suddenly going to get some sort of mentor who is going to teach him to do the right things on the pitch is baffling. Theo Walcott has never improved this side of his game and Wenger has had him for 10 years. One of many examples of players how don't suddenly start changing their decision making.
I think coaches can improve qualities that players already have and give them confidence, I don't believe someone who watches them train every day can suddenly give them a football brain unfortunately.