If you can replace one talented high-wage employee with 3 college hires then do this a couple times statistically you're still going to retain some talented employees because not all can leave. Those that remain will train the others ou of necessity either directly by management or indirectly by a fear of missing timelines/ego/being the local expert/etc.
Management knows this and actively encourages this behavior because by the time the talented employee burns out and self-destructs, at least one trainee will be competent enough to keep things moving.