> The only way out of this, you can probably see, is some sort of external co-ordination, as is the case with most of these situations.

You lack imagination. You can eg just charge juniors for the training.

Either directly (which won't really work, because juniors almost by definition don't have a lot of money), or via a bond that they have to pay back iff they jump ship before a set number of years.

Have a look at how airlines and airline pilots pay for their expensive education.