Humans and automation coexist. It's not as if all the engineers that used slide-rules and graphing paper went homeless when AutoCAD was invented. Same with farmers and the tractor. Society gets richer as automation makes our labor more efficient.
I'm an automation engineer. Automation and labor are at odds. Yeah, automation doesn't completely eliminate jobs, but it takes a lot fewer people to do those jobs. As per your example with farmers and the tractor, agriculture went from employing 90% of people to <2%, most of whom are doing tasks which are generally resistant to automation. You do not find humans guiding horse-drawn plows right alongside the tractors, both coexisting in harmony; the horse population plummeted as they were rendered obsolete and the humans have moved onto other things.
The main impediment to automation is the ready availability of cheap labor. Automation requires large upfront investments and lots of technical skill on the part of management. A highschool dropout has no upfront cost and responds to voice commands. I can say from personal experience, even when automation will easily pay for itself in a few months, there is generally still huge resistance to making that investment.
Throughout history you will find that many civilizations such as Rome or China had the opportunity to industrialize but didn't because labor was abundant. It is when there is a shortage of labor, such as after the black death, that moneyed interests are forced to invest in making their assets more productive. By paying people not to work (or more accurately not to work the undesirable jobs which ought to be automated) you get the same incentive to automate that a massive plague would induce, but without the tens of millions dead.
But you're proving my point here. 90% was pushed out of farming, but that population isn't unemployed right now. We're doing other things, like being automation engineers.
And as we automate other things, life will continue to improve, but humans won't ever run out of things to do.
People weren't pushed out of farming, people abandoned farming. The large investments in mechanization were made after migration to cities made it increasingly difficult to find low wage workers to tend farms. The automation wound up being much better than the humans it replaced, allowing agricultural productivity to skyrocket even as its labor force plummeted, but despite being the biggest beneficiaries, the farm owners had spent decades resisting these improvements. The average time from when a new farm mechanization was invented to when it started being commercially adopted was about 30 years. Progress begins with change.
No one is arguing that humans are at risk of running out of things to do, nor is that at all relevant to a discussion of UBI. No one is going to leave a fulfilling and rewarding job to live around the poverty level on UBI, UBI is about getting people out of the low paying, menial work that should have already been automated were it not for the glut of cheaply available labor, the jobs people only take because they will starve to death if they don't. The moment you take away the risk of starving to death, these people can do things besides burn away their hours - they can learn new skills, move to areas with more opportunities, or maybe just work on themselves, and then opportunities which were previously out of reach become available. Not everyone will necessarily find something better to do, but whereas under our current system these people would clog up the gears of progress by continuing to work automatable jobs, with UBI they are taken care of while society advances.
Note that UBI should not be 'paying people not to work'. It is paying people regardless of whether they work. The key to UBI being succesful (imho) is that everyone gets it and everyone gets the same amount (maybe excepting people under 18?). This makes the system so much simpler to administer.
It also prevents weird traps where doing something unintentionally costs you your UBI. (e.g. people who start working and earn too much to qualify for food-stamps / benefits / some other program, thus having them end up with less money each month).
Humans and automation coexist. It's not as if all the engineers that used slide-rules and graphing paper went homeless when AutoCAD was invented. Same with farmers and the tractor. Society gets richer as automation makes our labor more efficient.