Brain drain.

If you fire all your SWEs they won't sit around twiddling their thumbs waiting for an AI collapse, they'll career shift. Maybe to an unemployment line and/or homelessness, maybe to something else productive, but either way they'll lose SWE skills.

If you close down all the SWE junior positions you'll strongly discourage young people training in the field. They'll do something else.

Then if you want to go back, who will you hire for it?