I think some players don't really come of age until they have had loans, and a club often does them a disservice by having them on the cusp but never quite hitting a consistent stride. Look at Patto after his trip to Notts county, but more generally The likes of Kane and Beckham when they were young. People remember Beckham being young when he came into the Man Utd side, but only really matured at Preston if I remember rightly.
Even Amad - his career was going nowhere until he came to us, and save for his injury, I suspect he'd be a man utd regular now.
It would be nice to keep Johnson this summer, because he is clearly one of the most promising youngsters. But we do him and the club a disservice if we don't send him out on loan to somewhere where he could excell. I'd suggest somewhere like Birmingham might be perfect.