When a sector collapses and become irrelevant, all its workers no longer need to be employed. Some will no longer have any useful qualifications and won't be able to find another job. They will have to go back to training and find a different activity.
It's fine if it's an isolated event. Much worse when the event is repeated in many sectors almost simultaneously.
Why? When we've seen a sector collapse, the new jobs that rush in to fill the void are new, never seen before, and thus don't have training. You just jump in and figure things out along the way like everyone else.
The problem, though, is that people usually seek out jobs that they like. When that collapses they are left reeling and aren't apt to want to embrace something new. That mental hurdle is hard to overcome.
That means either:
1. The capitalists failed to redeploy capital after the collapse.
2. We entered into some kind of post-capitalism future.
To explore further, which one are you imagining?
Yes, we've had full employment for a long, long time. But the idea here is that AI will free up labor that is currently occupied doing something else. If you are trying to say it will fail to do that, that may be true, but if so this discussion is moot.
Many, many industries and jobs transformed or were relegated to much smaller niches.
Overall it was great.