Even if we make an assumption that it IS a zero sum game (and that there are no other jobs), then we're into a wildly different conversation around how we deal with allocating finite resources in a culture that supports property rights, and no one is going to solve that in a hacker news comment chain.
That is not automatically guaranteed.
Part of the problem with the Western economies is that the mills/mines/etc. used to employ a LOT of people.
A good example is the Natrona Heights steel plant. It used to employ something like 10,000 people at its height. After automation, it will now employ a couple hundred.
Where do 9,500 people switch employment to?
Take your example of the steel plant. Presumably when the 9500 people were laid off, productivity remained the same (or increased) and costs went down. Where did the extra profit go? Was it dispersed among the remaining 500 workers? Was it used to help the displaced workers who devoted years building the steel mill and keeping it running? The very people who designed, forged, and brought on-line the machines which replaced them? No, the upside went to the owners, because they have a slip of paper that says they own the mill.
Our problem is not that we're automating away jobs, it's that the displaced workers don't get to share in the benefits of that automation. Eliminating a job is seen as a disaster, rather than a victory. We should want to eliminate terrible jobs, not keep them around just so someone can stay busy. And I guarantee you, people who don't have to work will find something to do. There's always work to be done, just not all of it is profitable (e.g. raising children, taking care of the elderly, volunteering, open source coding, etc.)
Ever heard of Carnegie Tech (CMU), the Wharton School, or Swarthmore college? They exist precisely because of the existence of massive profits from steel.
Of course UBI or other safety net would be a good way to avoid social upheaval as well.
I know it's not an apples to apples comparison with any proposed UBI implementation, but looking at the social upheaval caused by the COVID-19 quarantine and stimulus programs, I'd say cutting a check to people and leaving a void in their life where work used to be is a fertile breeding ground for aimless people who will be easily radicalized. It's not something I ever considered pre-2020 as a big proponent of UBI, but now I feel it's something that has to give us concern.
I don't know how to express this but it frustrates me a great deal.
Money is akin to a video game. It's not real. It's just a token that allocates work. It doesn't decide how much work there is. People decide. You can spend the same dollar an infinite amount of time and thereby create an infinite demand for work.
Therefore work becomes a philosophical or an existential problem. We work for our own benefit to satisfy our needs and desires. To do so we have to do "productive" work, meaning work that helps someone else satisfy their needs and desires by proxy. What if you have already done enough to meet the basic needs of everyone? Yes, we are reaching the edge of humanity. The realm of philosophers. Why do we live? To work? We already worked enough to live. Beyond this point all activity including work is meaningless. When you accept that life itself is meaningless then so is the work that you do to sustain yourself.
How have humans dealt with this in the past? They did wholly pointless work. The pyramids weren't built because they were needed, it's because rulers decided to build them. Another way is to simply start a war. Once there is urgency, politicians are ready to spend whatever it takes, no matter how pointless the war. The Chinese just build infrastructure and housing even before there is a need or purpose.
Here, I'll do it. I'll prove that there is no lack of work and it is purely a political problem: Climate change prevention requires the creation of a huge amount of jobs. Yes this one sentence is a bomb. How does more work, research and technological progress make us poorer? It doesn't. The whole climate change denial/skepticism thing can't be explained by economics because we will end up better off by preventing climate change and I am not even talking about the benefit of preventing climate change. Just the benefit of creating more work. The powerful combination of economics and climate should make any capitalist immediately support the effort.
The only people who actually benefit from denying climate change are those directly working in the fossil fuel industry. Germany once gutted 100k jobs in the renewable industry to keep 20k coal jobs alive. It's 100% not about economics.
The answer is automation, logistics and supply chain improvements so you can service more customers with fewer, but more specialized employees who can be paid a living wage.