Not to relitigate anything but I'm pretty sure DA was trying to build a cost-controlled bench to match with his superstars (two or three) but the problem is that young players rarely can contribute at the NBA unless they are really really good. So if Romeo or Edward or Waters are ever going to be NBA players (and they may not be), it may take a couple more years. Which is not on the same timeframe as the Jays.Not piling on, but I will push back on the notion that the Celtics had anything to do with the lack of development of Langford or the others you noted. It's fair to be frustrated at some of these misses, but some were just never going to be NBA players no matter what (Edwards, Waters, Fall, Young).
White, however, is on the same time frame as the Jays.
The other thing is that to develop young players, they mostly have to be given minutes in environments where they are able to make mistakes. BOS is not like that. Romeo may never be healthy to be a legit NBA contributor but the fact that he stood in the corner on offense and watched a lot of ISO ball for two years wasn't going to develop him.