The implication of your comment is that people only do productive work now because they are forced to, to avoid starvation. And that you think such a system is fine. Because if you thought people now do work which interests them, you'd still think that in a UBI world, and if you thought the current system wasn't fine, you wouldn't go straight to "but who can be forced to do horrible things if we take the forcing lever away? That would be bad".
This is not about being productive. Productivity on its own is meaningless - when I draw I can work really hard but the effects of my work are literally wothless from the point of view of society.
I believe that capitalism works only because people need incetives that are aligned with society needs. Otherwise they will work really hard on stuff that uses resources but will not be part of sustainable system.
