Coach, I actually agree with you completely on your first statement.
Most kids pitching in travel ball will never develop a movement pitch. A few will develop 1 or 2 and even fewer will be good enough (or desire) to continue on to college ball.
Your last statement is flawed IMO...
...and only pertains to the kids that will never have the capacity to develop a movement pitch.
You are correct. The ones that can do it should do it.