I think it's a fine line. I think you can get to a point in a player's minor league career where you have to say: He's a finished product OR this is the best he's got, flawed but playable. No amount of coaching or training is going to make him better. Experience MAY provide incremental improvement over time but leaps of improvement is probably done.
Once you reach that point, then it becomes sink or swim time. But, I think you usually only reach that point when the player get's close to being out of options.
I think it would have helped Simmons (and Francouer) tremendously if they spent a year or two more in the minor leagues getting the take sign and learning some plate discipline. It's possible the Braves looked at both and said that they were NEVER going to learn plate discipline so you might as well get them up and get what you can out of them. But, I don't think that is what happened. I think the Braves had an immediate need and rushed both to fill that need. And I think it has hurt both long term in their careers.
If Simmons was a routine .800 OPS guy, with his defense, over a career, if long enough, he had the potential to be HoF worthy. Without the bat, he generates highlight tape but will be mostly forgotten in 20 years.