It depends how you view success. I get the feeling that Barton isn't bothered about playing for England.
And so what if Barton has under-achieved because of lack of application and desire and also his attitude. Perhaps that gives him a greater insight.
Barton is just saying what all of us...
Two points. You don't need to be a world-class player to have an informed opinion on whether someone else is and I'd also argue that Barton has made the most of his potential.You could even say that he's over-achieved and in no small way because of the persona he has built.
And of course...