> Let's say 50% of all jobs are lost.

So now things cost an average of 50% as much. If you were spending $100 before, it's $50 now, and you have the other $50 to spend on something else. Which in turn creates a job doing the something else.

Consider all of the other times things like this have happened. The majority of existing jobs at the time of e.g. the industrial revolution were the occupations that no longer exist or can be done with less than half the original amount of labor. Why then has the modern unemployment rate typically been at <10%? Because then people get other jobs.

The absolute extreme case where all jobs are automated is the one where nobody needs to work because all work is automated and everything is abundant. In every other case, people have jobs doing the things that can't be automated.

If it costs half as much to produce something, is the result that people get more of those things or less? Do more people starve today or in the times before tractors and combine harvesters?

The transition period often sucks, because then you have a lot of people whose skills are misaligned with where the new demand is, but long term that goes away so the real question is what do you want to do about it in the short term.