I think the biggest injury to the hiring of junior devs happened after COVID made remote-work ubiquitous. It's a lot harder for a junior dev to get real mentorship, including the ambient kind of mentorship-by-osmosis, when everyone works alone in a sad dark room in their basement, rather than in an office with their peers and mentors.

The advent of agentic coding is probably punch #2 in the one-two punch against juniors, but it's an extension of a pattern that's been unfolding for probably 5+ years now.