I don't agree that this is the central value juniors provide. Its a nice tertiary value, but not why one hires them. I think the value is the later part of farming for new talent and just growing your team.

I still think the central issue is the economy. There are more seniors available to fill roles, so filling out junior roles is less desirable. And perhaps "replacing juniors with AI" is just the industry's way of clumsily saving face.