> 1) Does expertise atrophy if you are just a human-in-the-loop

Well I guess you use the same technique as for muscle athrophy. I sit all day at work and and drive to places using machines, so my body athrophies due to lack of movement. We built gyms where you go to to fight that athrophy. And we invented jogging and hiking where you go to places without the intent to actually travel somewhere. Same thing applies: You will have to program without AI on projects, that are not your real end product to keep your skills in training.

> 2) How do junior engineers develop the so-called expertise if they have not gone through all the steps and suffered repeatedly?

Same as above. The training + education will take longer, we will have to create training products for the juniors to practice and gain experience. The products will not necessarily be used in production, but should only be seen as sparring rigs.