If I were a betting man, I would wager that those managers either don't care, or are gambling that by the time senior engineers are in short supply, AI will be good enough to replace them as well.
Anyway as an average single company anything you do won't move the needle much - those people you train can move on anyway so that isn't a good way to cover that risk. Even if I have to pay more later, which is an unlikely outcome potentially given AI, that's tomorrow's problem and it affects my competitors and other companies as well most probably so I'm not at a relative disadvantage. Unless I'm a very big employer of tech I'm not going to affect future market dynamics either way as a single team.
However if the market is in structural decline and jobs will whittle away - maybe its better we don't hire juniors? They may thank us when they settle in another career with better long term prospects if what many posts are starting to say - that AI will kill the industry slowly. Better the hiring pool shrink to adjust to future expected demand than have higher unemployment and worse issues later.