Or their children, who didn't even work to get to that point. If unconditional cash grants are bad, outlaw inheritance; it's just an unconditional cash grant awarded by genetic lottery. If working for success it's so paramount, make everyone do it.
I'm pretty sure it's wrong, too. Every UBI trial I've seen has people improving their lives. Some get lifted into work. Some get necessary training. A very high percentage have their life and status improved, better than welfare.
Those who only ever act out of greed and selfishness always believe these are the only reasons anyone could ever be motivated to act. The notion that people could work because they like it, or because it helps others, or because they would rather be active than inactive, is out of reach of their imaginations.