[cornholio]

An alternate explanation is that, even if AI does not have any chance of entirely replacing an employee, not even junior hires, in the hands of competent seniors AI does substantially improve their productivity and eliminates whole classes of tasks that were traditionally where juniors cut their teeth.

So companies reduce junior hiring because their work is relatively less valuable, and they can meet their goals by shuffling existing resources. When they can't do that, they go for seniors since the immediate bang for the buck is higher (ofc, while depleting the global pipeline that actually produces seniors in the long run, in a typical tragedy of the commons)