Training people is a cost, an investment, if everyone does it the cost is amortized across the industry. If I can cut that cost by using AI I'm now at a competitive advantage, everyone will look to cut that cost because the downside of paying for training juniors that may leave is worse for that company than the downside of the whole industry not training juniors any more.
Fast forward half a decade forward, and there's no juniors or mediors to onboard, no-one to have gemeric expertise that can be fine tuned to your product.
The juniors you train today someone will poach, surely, as you'll poach someone else's, and it levels out.