> A company might as well spend months of their experienced developers' time while making them mentor the more junior devs and invest a lot of resources in this

> A company might as well spend months of their experienced developers' time while making them mentor the more junior devs and invest a lot of resources in this, as well as have more overhead to their development because of needing to spend longer on QA and fixing bugs introduced by these juniors, for them to just turn around and leave when they've gotten the seniority that they desire. There's simple solution to people leaving - just pay them above market rates. In my current company, a manager had this crazy idea, and what do you know - a lot of the key people he hired (most of them were excellent to begin with, as when you advertise above-market pay, you attract some pretty great people) are still with us 5 years later, and are being the pillars of their teams and of entire departments.