I'm also pretty sure in the past industrial transitions, many of the people who lost their jobs at the start of the change never found better ones. It took a generation or so for new opportunities to really be found and fine tuned and you're competing for those new roles with younger people anyway.

If ai does take a lot of white collar work, is it a lot of comfort that maybe jobs in a very different sector will be better in 20 years?

Did the younger people find better jobs? You used to have all these jobs for people who were maybe a bit smarter than average with good judgment. In the 1990s, the local community college used to advertise associates degrees for paralegals. That's a job that doesn't exist in the same way anymore thanks to computers. Now it's become an internship for kids with top credentials before they go to law school. Which is fine for them, but what about everyone else?

It seems to me like all of these people are flocking now to healthcare fields. That seems totally unsustainable.

Not in all past industrial transitions.

But yes, the argument has been wrong often enough that the people still repeating it as a rule should be mocked and ashamed.