It won't make it impossible for junior engineers to learn.

It will simply reduce the amount of opportunities to learn (and not just for juniors), by virtue of companies' beancounters concluding "two for one" (several juniors) doesn't return the same as "buy one get one free" (existing staff + AI license).

I dread the day we all "learn from AI". The social interaction part of learning is just as important as the content of it, really, especially when you're young; none of that comes across yet in the pure "1:1 interaction" with AI.