No one has ever said the ones who don't pan out don't count, but has there ever been an AHL who has lost his job because he failed to develop every player who came through his team into an NHLer? Has any AHL coach done that? Maybe it's more about setting expectations within the realm of what's actually possible, and realizing that he has a very high success rate here when compared to other AHL coaches.
The forwards he has worked with were either low upside in the first place or had already shown significant setbacks in their development before they got to him. While I'd of course like to see more of them turn into top scorers, that doesn't seem hugely realistic either.