> But to play devil's advocate for a moment, isn't there something of a natural disincentive to level up one's employees in such a tight labor market? To follow the author's analogy, by mentoring someone, we're effectively "adding a fish" to the pond, and if someone else "catches" the fish, that effort is largely wasted.

Most good employees don't quit for at better job unless there's a dysfunctional workplace. Partly they don't know if there's better jobs, and partly it's just a lot of effort, but mostly because other companies don't know they're good employees. The labor market is a market for lemons. If you own a $2000 car that drives OK you don't sell it because other people won't believe it's any good.