Ask HN: Why do people assume UBI will work?

There's been a lot of exploration on the subject of UBI lately and there seems to be steadily increasing agreement that it's the only viable approach in the long run.

But there are something like 3B+ people in the world who presently live on $2.50/day or less. Even adjusting for cost of living, one is forced to conclude this income level is still just barely better than outright barbarism.

So why is UBI considered viable? What do we think is going to change about how human societies are structured between now and the future that will lift some lucky subset of (presumably first-world) citizens above a CoL-adjusted $2.50/day? Or do we assume UBI will happen and will leave people at just barely survivable income levels?