The idea that there are lots of great engineers out there who just need mentoring and guidance to excel sounds great, but the reality of the industry seems to be that people dont stay in one place very long.

I think there is an opportunity there, but you would need to use contracts or something to make sure the time and effort you are putting into making this person great isnt just a gift you are giving to the company they move to next year. Also, I dont think startups are at all well placed to do this, given their time pressures. Google, Apple and Facebook could totally be doing this though.