It's a great idea but the reason why this won't work is because after you have invested time to train them from nothing, they will leave for another company.
There is plenty of precedent for this, during the dotcom days and in India the last 10 years.
Exactly this, people leave for other reasons and generally it isn't because you're investing in them. Sure it happens but it isn't the usual case in my experience.
It doesn't have to be that bad. There's an old bit about the teacher learning more than the students.
I've known plenty of master programmers who could tell me nothing about mastery other than 'train hard, train long'. I think actually this comes from 'sacrificing' everything else in life to become good coders at a young age, and we don't take the time to see what mastering other skills looks and feels like. I call it "Master of one, master of none."
Every time you teach something you should get a little better at teaching it. Your materials should get clearer. Your understanding of the material should improve. At the end of the day you and your training material are better for having had the student, even if they bail on you just when they're getting interesting. But if you respect the student and the exercise I don't think you'll have that happen that often, unless there's something fishy going on elsewhere in the organization.
Conversely, I think going off and earnestly learning a martial art, cooking ('mise en place' is a concept every coder should know), an instrument, or Chess or Go, or preferably at least two of those, will do wonders not just for your coding but for your personality. First, you will have to get comfortable with being bad at something again. Then you will watch yourself think you have something figured out, and then the teacher will tear it all down and build you back up at a new layer, only to tear it all down again a year from now. Watching that rug get pulled out from underneath you over and over again makes you start wondering what other things in life you're comfortably dead wrong about, because you believed in a colorful fiction that was 'true' for you where you were but is not the objective truth. And maybe the objective truth doesn't matter because it benefits almost nobody anyway. There's just where you are and where you're going.
There is plenty of precedent for this, during the dotcom days and in India the last 10 years.