Who is going to teach the junior developers?

What about them?

Juniors are extremely expensive to train and teach and AI gets rid of this position.

The junior engineer position is dead and most companies don't need junior engineers at all.

I would also say the same for senior engineers, but there will be less of them in the future.

My advice to new engineers out of school or looking for their first job:

Do not call yourself 'junior', you have AI now as well as the 'seniors', except they are too comfortable and don't have the drive as juniors do.

Redirect that drive and make your own business or startup, you'll learn a lot more doing that.

And even if that doesn't work out, there will be always new jobs for humans to do.

There won’t be any to teach

I taught myself (early 80s, pre-internet or pre-BBS). These days the resources are infinite. You don't need a "teacher".

Be curious. Take initiative. Try things.

gAI