I'd argue that this is trying to paint a very black and white picture in an area that has many more shades of grey. Training takes time and resources away from making the game and is an investment. Less training time equals more time spent making the game.
Both seniors and juniors should always be learning, and becoming better. It requires a balance, and the ratio of junior/senior can only support so many juniors. The investment can totally be worth it, if you have time for it to pay off.