We know how this goes, at least in broad strokes. We've been through it before, just not with "knowledge" work. If there is demand for something then supply will show up to provide it.

Some jobs will stick around in vastly diminished numbers with tasks that are completely different than what they used to be to produce the same output (e.g. farmer). Other jobs will be eliminated entirely (e.g. switchboard operator). I'm guessing things like software engineering will go the way of the farmer, with the main unknown being just how much demand for software there is.

Why do people think that people displaced by AI in one industry will get a job in another industry?

The entire valuation of the AI industry is predicated on people not just losing individual jobs, but being taken out of the workforce entirely on an economic level.

They are talking about the workforce of the entire economy shrinking. People will lose their livelihoods for good.

This has the potential to be even worse than the second agricultural revolution to industrial revolution phase, which made ordinary workers lives absolutely miserable for maybe a hundred and fifty years.

This time, there will be no jobs. If you are displaced from one industry by AI, you will end up in another industry also being decimated by AI; if you get a job at all, you will do so by working lower pay than other workers, who will in turn be pushed down the ladder.

And that is if you are lucky: if you have only IT skills, why should you be the first to get a fruit picking or plumbing job?

I would argue that how it goes ends up with the main downstream effect of shifting knowledge growth and therefore expertise and therefore economic power away from the people who outsource it and to the people who do that outsourced work.

In this case with AI that power will shift to the companies that run the AIs