10 years from now, the people that stopped hiring novices and juniors are going to be deeply regretting their past decisions. The people that kept hiring are going to be working with their newly-promoted-to-senior colleagues and be making significantly more progress than those that didn’t keep hiring.
Will they? Just because you developed them that doesn't guarantee they will stay with you. It's been always the same issue tbh, but big companies could accept the risk because they pay the most competitive salaries anyways.
Ten years from now the people making these decisions have moved on to different companies and cashed their quarterly bonuses.
Except they won't. They will just hire those new people away from the firms that trained them. That's what happens now and there's no reason why it won't happen in the future.
This is why firms that do actual training have clauses written in the employment contract that says if you receive x months of training from them then you have to work for them for at least y number of years otherwise if you leave then you have to pay them for the cost of training you (which is written as a dollar amount in the contract).
Companies that don't have that kind of clause in the contract are going to get screwed over when their newly trained employees get poached by other firms.
(IBM figured this out a couple of months ago, and explicitly announced tripling their hiring of juniors/grads in order to avoid ending up with a massive gap in the management/senior layers in future).