It’s a Volunteer’s Dilemma: why train juniors when you can hire the juniors other companies trained to become seniors?
In centuries past apprentices would pay their junior positions, in time picking up paid work as the progressed to senior, then eventually taking on apprentices of their own (and be paid)
I guess universities will eventually backfill that gap with training that focuses on senior skills. Eventually.
We already require juniors to go through a 4 years university degree. It takes a fair bit of time of real world work to get to the senior level. So unless we expect people to do another 5 years of schooling, I am not sure how this will happen (and even in that scenario I believe there is a difference between hands on, on the job experience and classroom experience).
I suggest that the first ladder that got pulled up is the one on the ground.
No one wants to train new entrants to the field. Not training junior workers seems like a natural extension to that.