I disagree that training juniors keeps them loyal. When you bring someone the island of misfit toys and they get some experience and success, they no longer see themselves as a misfit toy. But when they come in to the office in the morning, they look all around them and see misfit toys. But I am not a misfit toy any more! I can leave the island now!
Misfit toys are hungry people! They want to prove that they can do it, since by all other metrics they should not be able to. And once they have proven they can do it, they leave to find better appreciation of their talents.