Actually, the cost of mentoring is not necessarily only cost of lost productivity. Sometimes (I hope, more likely mostly) there are pieces of old, arcane, barely and wrongly documented pieces of codebase that there is exactly one senior in the company who knows how it works. Assigning said senior from coding literally means pushing deadlines. So it is kind of tragedy of the commons.
That's like, a great motivation to assign said senior AWAY from coding! "Single points of failures" are your greatest enemy, your senior my get a better offer. Or might get sick. Or might get in a car accident. Why would you want to bet your entire company on a single person? I mean... sometimes you have to do it because there's no other way, but it's a big warning sign, and you should diligently work to get out of that situation ASAP. "Pushing deadlines" might very well be a price worth paying in a situation like that.
One of the good reasons to have junior devs around is that (if they're motivated) they'll find holes in the documentation and help prevent this. I usually write many pages of text to document problems and unwritten ideas at my internships.
This is probably the biggest difference between companies who excel and those who don't. Some companies get that employees are assets and training them is worth it even if they might leave.
