That is the value of other companies doing that and you going to poach those new seniors. With the money you saved not training those juniors you can offer better salaries and still have higher profits.

Their main value is in being cheap before they realize that they're underpaid and hop jobs.

They tend to catch on quicker these days, making companies more reluctant to hire them. It has little to do with AI.

I want to agree with you but when was this ever true?

It was true when I started to work in this industry ~8 years ago. But of course YMMV especially depending on country and company