I think he does a better job that we - or at least I - have given him credit for. I used to regard him as a waste of space, but the more I hear the more I feel that he does pretty much what you'd want him to do. He does identify the weaknesses and he does work on them with each keeper. It makes me more inclined to doubt what any coach can do if the keeper is a bit thick and/or unable to change their instincts. What could another coach add? Motivation, I guess - the ability to inspire so-so keepers to excel themselves. Greater discipline and precision - doing what Achterberg does but better than he does it. But I'm getting increasingly sceptical about how much you can improve keepers - you either find a really good one, who is self-motivating and barely in need of coaching as such, or you don't, and you have to try to limit the damage.
As for top keepers becoming top coaches, I don't think there's any real connection. Clemence was England keeper coach and the cock-ups continued. I think only Bob Wilson had a long career as a specialist coach in this country and again he looked good when Seaman was keeper and not good when the other keepers played.