The "not hiring juniors" thing started before agentic LLMs were a thing, but it was close enough that it's easy to mistake it for causation. The fact that people retroactively attribute this explanation to an industry trend just reinforces it in people's minds, otherwise people might have tried to hire juniors again and realized that it's not such a bad idea. By not hiring juniors, you're putting mid-level work on seniors, who could otherwise be dedicating themselves to solving hard-and-expert-level problems. You're handicapping your company.