What does UBI boil down to? It's a prop to keep capitalism from running off the rails once society no longer carries the structures that made it a stable social arrangement.
But it needs to do more than that to work in the long haul.
It needs to support the current economic system and it needs to suppress the reorganization of society around whatever new social structures emerge. Because those laborers are indeed human beings. If they're freed up, they're going to go into society and create new complexity.
And that newness is going to give rise to something else.