Wow, good topic and I have to go with Deldad.
I think it depends on the player. For example, for pitchers, they can't rely on velocity alone.
They need to also develop the other tools needed for pitchers to get to the next level.
Same with position players, let's say if the player hits for power, but doesn't field his position well, he has to work on that tool, etc.
You have to work hard to try to develop all areas, for your position, for the next level. You do not have to be the best at everything.
As players get older and mature, their strengths and weakness become more apparant. In our case, son's arm was his strength, he could hit but not for power, and was slower than a turtle trying to get on base.
So it was apparant he would become a pitcher, so he focused on what needed to be done to develop for that position. He was not happy about this at first, but he had to go with what apparantly were his stronger tools.
I know of a player who could hit just about anything you gave him. He had been a very good pitcher before he needed arm surgery and had to give up pitching. When he went into a slump in HS(long one), he became less attractive because he had fair speed and did not field his position very well. Parents had spent lots of money on hitting lessons after the arm surgery, but I am not sure if anyone paid attention to what was going on in the field.
JMO.