Undiscovered: What you were told is flat out wrong re: any breaking ball.
Muscle development has every thing to do with how certain pitches would affect a young arm. Perhaps more importantly then that; coordination is also a big factor and furthermore the mechanical approaches that are needed in order to not cause undue stress on any pitcher at any age by throwing a breaking pitch incorrectly can be damaging.
Location, changing speeds and not pitching from behind are the most important factors to being a successful pitcher at any level.
Lastly a qaulified coach at any level above high school can teach a kid a breaking ball in about ten minutes and have him throwing it in a game by the end of the week.
My point is when I an NCAA Division II pitching coach the first thing we did with most of our pitchers was get rid of the high school breaking ball especially for our right handers because at that level it would no longer work…righty to righty the ball was breaking right towards the bat head unless the RHP had a true 12/6 but for RHP this is rare.
What I am getting at is there are not many college coaches or professional ones for that matter who would be un happy with a kid who has a potential but no breaking ball because they will get to teach the pitcher to throw it correctly instead of fixing something that is wrong and will take far more time.
At the professional ranks if a scout is looking at a projectable kid with absolutely no breaking ball what so ever he is not going to care at all if he has other desirable factors in terms of projectability. Matter of fact this particular kid may be a better investment over the long haul.
How much better is it for all youth pitchers to master command of the fast ball first, then varying speeds of the fast ball, followed by the change up? My answer to the hypothetical would be, “WAY BETTER!” due to a plethora of reasons.