My thought is only that players need playing time to develop. Coaching factor may be somewhat overrated because we don't have enough sample of owners who send all their prospects immediately where their best coaches are. We probably do have enough sample - absentee owners, dead in the water teams - to show that poor coaches and management will hinder development.
The concept of "rushing" might come from the fact that young players playing at a level appropriate to their ratings often struggle with production at first. But it makes sense because when we do this we're putting a player at a level higher than where he can dominate. I only once ever put a 21 year old player in the bigs, and it was because he was the best player available to me. He was ready, and he was better than anything else I had, and he was better than any free agent I could have signed. And he developed fine, highest rated position player I've ever had.