Training one position,if you not have some player with high potential,the older players around 21-22 years old stop
So,you however would waste time training players at the potential cap.
You don't say a wrong thing,but at least when you change training ,you have to go through a pair of season in which you had to train 2 or 3 players 18-19 years old,and you have to put one of them in the line-up,then you could go with the tactic that you said in the previous posts ;D